A Solarized Dark theme for Mathematica. I have not created a Solarized Light theme, but it should be quite straightforward to implement (and if you do, please consider submitting a pull request ☺).
The stylesheet file is SolarizedDarkStylesheet.nb and is generated from
SolarizedDark.nb. If you wish to change anything, you can do so in
SolarizedDark.nb and recreate SolarizedDarkStylesheet.nb. In particular,
you may consider changing the fonts.
The theme mostly works, though there are a few components which don't fit in
with the theme (mostly, dynamic/interactive content, and objects with special
formatting such as SparseArray and Entity).
I am no longer using it much as I have opted for another theme that resembles more closely the default theme. You are welcome to use it, and please feel free to submit pull requests if you wish to change things.