I was inspired by this post so I've made a Solarized stylesheet after mucking around with the text options and asking stupid questions on here. I know you can change the syntax highlighting by running option-setting commands , but I'd rather have a stylesheet that takes care of all that stuff.
Here is the link to a notebook with the solarized stylesheet. After you open the notebook, you can access the stylesheet by going to Format > Edit Stylesheet and follow my instructions on there on how to modify the stylesheet to fit your needs, while observing the changes in the notebook as you change the stylesheet. One way to learn how to edit stylesheet is to look inside a style cell (select the cell and press Ctrl+Shift+E) and modify the parameters in there--such as color paramters in RGBColor[x,y,z]--and see what your notebook looks like after you exit the cell view (Ctrl+Shift+E again).
You can switch between light and dark theme by going to File > Screen Environments and choose between Working (light) and Dark (dark) themes. When you're happy with the way it looks just install/add the stylesheet by clicking on the "Install Stylesheet..." button and choose the Source as the "NotebookObject", then choose a name and if you'd like to install it for just you or for all users of your computer. After that, you can apply the stylesheet to any notebook you'd like by going to Format > Stylesheet, and choose the name of the stylesheet you saved it as.
I know the steps are pretty obvious but as a newbie to MMA I really appreciated the people who gives out step-by-step instructions, and I'd like to do the same here. Please let me know if you have any trouble testing and installing the stylesheet and I could help you if I'm able to. I do appreciate any feedback you guys might have. Do you find the color scheme too much? What do you think could be improved upon it?
Enough talk: some examples of the theme, and there's also more examples in the notebook in the link.