There are a few user interface changes in Mathematica 11, and the new font is what I'm interested in. Unfortunately, the official document doesn't say what the font's name is.
I attempted to find the answer using this command:
In[1]:= System`$DefaultFont
Out[1]:= {"Courier", 10.}
which is obviously the same result as in version 10 (see here). I don't think Courier is the right answer.
Some comments below suggest the answer varies from one platform to another. My system is Linux Mint 17.3 KDE. I'm sure the font isn't Source Code Pro, although it does exist in my font folder. It should be one of DejaVu Sans Mono Bold and Bitstream Vera Sans Mono Bold, but I can't tell exactly which one is correct, because they look identical:
Comments are welcomed and if you have a different font, please attach a sample screenshot in your answer.
CurrentValue[{StyleHints, "CodeFont"}]
$\endgroup$Default font changed to Source Code Pro on Mac, Consolas on Windows, and Bitstream Vera Sans Mono on Linux.
$\endgroup$