How do I install a color theme for IntelliJ IDEA 7.0.x

Go to File->Import Settings... and select the jar settings file

Update as of IntelliJ 2020:

Go to File -> Manage IDE Settings -> Import Settings...

Step 1: Do File -> Import Settings... and select the settings jar file

Step 2: Go to Settings -> Editor -> Colors and Fonts to choose the theme you just installed.