To view Theme files for ST3, install PackageResourceViewer via PackageControl.
Then, you can use the Ctrl + Shift + P >> PackageResourceViewer: Open Resource to view theme files.
To edit a specific background color, you need to create a new file in your user packages folder Packages/User/SublimeLinter with the same name as the theme currently applied to your sublime text file.
However, if your theme is a 3rd party theme package installed via package control, you can edit the hex value in that file directly, under background. For example:
<dict>
<dict>
<key>background</key>
<string>#073642</string>
</dict>
</dict>
Otherwise, if you are trying to modify a native sublime theme, add the following to the new file you create (named the same as the native theme, such as Monokai.sublime-color-scheme) with your color choice
{
"globals":
{
"background": "rgb(5,5,5)"
}
}
Then, you can open the file you wish the syntax / color to be applied to and then go to Syntax-Specific settings (under Preferences) and add the path of the file to the syntax specific settings file like so:
{
"color_scheme": "Packages/User/SublimeLinter/Monokai.sublime-color-scheme"
}
Note that if you have installed a theme via package control, it probably has the .tmTheme file extension.
If you are wanting to edit the background color of the sidebar to be darker, go to Preferences > Theme > Adaptive.sublime-theme
This my answer based on my personal experience and info gleaned from the accepted answer on this page, if you'd like more information.
Packagesfolder for ST3 and ST2 are different; ST3 begins with only aUserfolder. Thank you for the reference link, but it doesn't explain how to make a theme or how to make it change themes per file type. - Luke