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.

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sublime-monokai-sidebar

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:

Installation via Terminal

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

Installation using other methods

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.