Thank you for the kind words
Unfortunately it's not possible with current approach. You have to tweak it manually:
1) Unpack theme via PackageResourceViewer.
Packages/Boxy Theme/Boxy Monokai.sublime-theme file.
3) Find something like this:
// Sidebar Entry
"color": [151, 151, 148] // Red, Green, Blue
4) Tweak color value & save.
P.S. Currently I'm working on a new adaptive theme with better settings management. I hope I'll finish beta version at the end of July. So keep in touch, your issue will be resolved soon.