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Here's a way to do it. First we'll use this as our DockedCell expression:

newDockedCell =
  Cell[
   BoxData@ToBoxes@
     Pane[
      Import["https://i.imgur.com/LUIaEdC.png"],
      ImageSize ->
       {
        FEPrivate`Part[
         FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
          WindowSize],
         1],
        Automatic
        },
      ImageSizeAction -> "ShrinkToFit",
      Alignment -> Center
      ]
   ];

Then just add these style edits to the stylesheet however you like. This is my preferred short-hand (using custom package-level functions), but the easiest thing is just to do it manually as you have done:

StyleSheetEdit[
 "Notebook",
 DockedCells ->
  newDockedCell,
 "MakeCell" -> True
 ]

StyleSheetEdit[
 "DockedCell",
 {
  CellFrameMargins -> 0,
  Background -> White
  },
 "MakeCell" -> True
 ]

The key here is this:

FEPrivate`Part[
 FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
  WindowSize],
 1]

That will be computed by the front-end directly and give the notebook width.

By setting these:

ImageSizeAction -> "ShrinkToFit",
Alignment -> Center

The Pane will force the content to be centered and take on that size and shrink if constrained.

You can see from these that it works:

Getting this into the stylesheet:

This can be a little annoying to add to the stylesheet, so here's an effective way to do this:

• Go to the stylesheet
• Find the style CellObject for "Notebook". This can be most easily gotten via PreviousCellorNextCell`.
• Run this:

SetOptions[thatCell, DockedCells->newDockedCell]

• Select that cell and toggle Show Expression on and off

It should work after that

Here's a way to do it. First we'll use this as our DockedCell expression:

newDockedCell =
  Cell[
   BoxData@ToBoxes@
     Pane[
      Import["https://i.imgur.com/LUIaEdC.png"],
      ImageSize ->
       {
        FEPrivate`Part[
         FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
          WindowSize],
         1],
        Automatic
        },
      ImageSizeAction -> "ShrinkToFit",
      Alignment -> Center
      ]
   ];

Then just add these style edits to the stylesheet however you like. This is my preferred short-hand (using custom package-level functions), but the easiest thing is just to do it manually as you have done:

StyleSheetEdit[
 "Notebook",
 DockedCells ->
  newDockedCell,
 "MakeCell" -> True
 ]

StyleSheetEdit[
 "DockedCell",
 {
  CellFrameMargins -> 0,
  Background -> White
  },
 "MakeCell" -> True
 ]

The key here is this:

FEPrivate`Part[
 FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
  WindowSize],
 1]

That will be computed by the front-end directly and give the notebook width.

By setting these:

ImageSizeAction -> "ShrinkToFit",
Alignment -> Center

The Pane will force the content to be centered and take on that size and shrink if constrained.

You can see from these that it works:

Getting this into the stylesheet:

This can be a little annoying to add to the stylesheet, so here's an effective way to do this:

• Go to the stylesheet
• Find the style CellObject for "Notebook". This can be most easily gotten via PreviousCell or NextCell.
• Run this:

SetOptions[thatCell, DockedCells->newDockedCell]

• Select that cell and toggle Show Expression on and off

It should work after that

Here's a way to do it. First we'll use this as our DockedCell expression:

newDockedCell =
  Cell[
   BoxData@ToBoxes@
     Pane[
      Import["https://i.imgur.com/LUIaEdC.png"],
      ImageSize ->
       {
        FEPrivate`Part[
         FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
          WindowSize],
         1],
        Automatic
        },
      ImageSizeAction -> "ShrinkToFit",
      Alignment -> Center
      ]
   ];

Then just add these style edits to the stylesheet however you like. This is my preferred short-hand (using custom package-level functions), but the easiest thing is just to do it manually as you have done:

StyleSheetEdit[
 "Notebook",
 DockedCells ->
  newDockedCell,
 "MakeCell" -> True
 ]

StyleSheetEdit[
 "DockedCell",
 {
  CellFrameMargins -> 0,
  Background -> White
  },
 "MakeCell" -> True
 ]

The key here is this:

FEPrivate`Part[
 FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
  WindowSize],
 1]

That will be computed by the front-end directly and give the notebook width.

By setting these:

ImageSizeAction -> "ShrinkToFit",
Alignment -> Center

The Pane will force the content to be centered and take on that size and shrink if constrained.

You can see from these that it works:

Getting this into the stylesheet:

This can be a little annoying to add to the stylesheet, so here's an effective way to do this:

• Go to the stylesheet
• Find the style Cell for `"Notebook"
• Run this:

SetOptions[thatCell, DockedCells->newDockedCell]

• Select that cell and toggle Show Expression on and off

It should work after that

Here's a way to do it. First we'll use this as our DockedCell expression:

newDockedCell =
  Cell[
   BoxData@ToBoxes@
     Pane[
      Import["https://i.imgur.com/LUIaEdC.png"],
      ImageSize ->
       {
        FEPrivate`Part[
         FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
          WindowSize],
         1],
        Automatic
        },
      ImageSizeAction -> "ShrinkToFit",
      Alignment -> Center
      ]
   ];

Then just add these style edits to the stylesheet however you like. This is my preferred short-hand (using custom package-level functions), but the easiest thing is just to do it manually as you have done:

StyleSheetEdit[
 "Notebook",
 DockedCells ->
  newDockedCell,
 "MakeCell" -> True
 ]

StyleSheetEdit[
 "DockedCell",
 {
  CellFrameMargins -> 0,
  Background -> White
  },
 "MakeCell" -> True
 ]

The key here is this:

FEPrivate`Part[
 FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
  WindowSize],
 1]

That will be computed by the front-end directly and give the notebook width.

By setting these:

ImageSizeAction -> "ShrinkToFit",
Alignment -> Center

The Pane will force the content to be centered and take on that size and shrink if constrained.

You can see from these that it works:

Getting this into the stylesheet:

This can be a little annoying to add to the stylesheet, so here's an effective way to do this:

• Go to the stylesheet
• Find the style CellObject for "Notebook". This can be most easily gotten via PreviousCellorNextCell`.
• Run this:

SetOptions[thatCell, DockedCells->newDockedCell]

• Select that cell and toggle Show Expression on and off

It should work after that

Here's a way to do it. First we'll use this as our DockedCell expression:

newDockedCell =
  Cell[
   BoxData@ToBoxes@
     Pane[
      Import["https://i.imgur.com/LUIaEdC.png"],
      ImageSize ->
       {
        FEPrivate`Part[
         FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
          WindowSize],
         1],
        Automatic
        },
      ImageSizeAction -> "ShrinkToFit",
      Alignment -> Center
      ]
   ];

Then just add these style edits to the stylesheet however you like. This is my preferred short-hand (using custom package-level functions), but the easiest thing is just to do it manually as you have done:

StyleSheetEdit[
 "Notebook",
 DockedCells ->
  newDockedCell,
 "MakeCell" -> True
 ]

StyleSheetEdit[
 "DockedCell",
 {
  CellFrameMargins -> 0,
  Background -> White
  },
 "MakeCell" -> True
 ]

The key here is this:

FEPrivate`Part[
 FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
  WindowSize],
 1]

That will be computed by the front-end directly and give the notebook width.

By setting these:

ImageSizeAction -> "ShrinkToFit",
Alignment -> Center

The Pane will force the content to be centered and take on that size and shrink if constrained.

You can see from these that it works:

Here's a way to do it. First we'll use this as our DockedCell expression:

newDockedCell =
  Cell[
   BoxData@ToBoxes@
     Pane[
      Import["https://i.imgur.com/LUIaEdC.png"],
      ImageSize ->
       {
        FEPrivate`Part[
         FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
          WindowSize],
         1],
        Automatic
        },
      ImageSizeAction -> "ShrinkToFit",
      Alignment -> Center
      ]
   ];

Then just add these style edits to the stylesheet however you like. This is my preferred short-hand (using custom package-level functions), but the easiest thing is just to do it manually as you have done:

StyleSheetEdit[
 "Notebook",
 DockedCells ->
  newDockedCell,
 "MakeCell" -> True
 ]

StyleSheetEdit[
 "DockedCell",
 {
  CellFrameMargins -> 0,
  Background -> White
  },
 "MakeCell" -> True
 ]

The key here is this:

FEPrivate`Part[
 FrontEnd`AbsoluteCurrentValue[FrontEnd`EvaluationNotebook[], 
  WindowSize],
 1]

That will be computed by the front-end directly and give the notebook width.

By setting these:

ImageSizeAction -> "ShrinkToFit",
Alignment -> Center

The Pane will force the content to be centered and take on that size and shrink if constrained.

You can see from these that it works:

Getting this into the stylesheet:

This can be a little annoying to add to the stylesheet, so here's an effective way to do this:

• Go to the stylesheet
• Find the style Cell for `"Notebook"
• Run this:

SetOptions[thatCell, DockedCells->newDockedCell]

• Select that cell and toggle Show Expression on and off

It should work after that