This is not being actively developed nor supported. If you are still interested in it, you can check the Sublime Text 3 documentation for themes, or look for complete themes, there are some good ones out there.

Sublime Text is great, but it's sidebar doesn't match the default (and beautiful) dark Monokai theme. Thanks to this gist and after tweaking it a little (I corrected a glitch with the code collapse icon and made the higlight color lighter), I finally made it look great:

macOS:

cd sublime-monokai-sidebar/

mkdir /Users/$USER/Library/Application\ Support/Sublime\ Text\ 3/Packages/Theme 

mv Default.sublime-theme /Users/$USER/Library/Application\ Support/Sublime\ Text\ 3/Packages/Theme/

Linux:

cd sublime-monokai-sidebar/

mkdir /home/$USER/.config/sublime-text-3/Packages/Theme

mv Default.sublime-theme /home/$USER/.config/sublime-text-3/Packages/Theme

Windows (File Explorer):

Copy file to user directory here:

%userprofile%\AppData\Roaming\Sublime Text 3\Packages\User\Theme

Using Sublime Text 3:

Press ctl + shift + p or shift + cmd + p to open Command Palette, type Browse Packages and hit enter. In that folder, create a Theme directory and place the Default.sublime-theme file.