Simple solution for SublimeText 3 - change active tab title font color:
1) copy Theme - Default.sublime-package from /Packages folder to other place
2) rename file - add .zip extension and extract archive to some folder
3) find Default.sublime-theme file and open it in text editor
4) find **// Tab Labels ** section
5) for each tab_label entry with selected attribute change "fg" color from "255, 255, 255" to e.g. "255, 216, 0"
6) save file and pack folder to Theme - Default.sublime-package.zip (be careful - don't create subfolder level)
7) remove .zip extension and put Theme - Default.sublime-package into /Packages folder (rename original file for backup purpose to e.g. Theme - Default.sublime-package.bak)
8) run SublimeText and enjoy!
Probably is possible to replace tab backgrounds (in archive Theme - Default.sublime-package.zip
you can find all needed files), but it's embarrassing without documentation.