In OmegaT 5.6 or later:
plugins
in your user configuration folder (which you can open from OmegaT going to Options → Access configuration folder).See the plugins page for available theme plugins.
Changing colors inside the panes can be done through Preferences > Appearance > Colours.
On Mac, go to System Preferences > Language and Region > Apps, click on the [ + ] button at the bottom and add OmegaT.app to the list along with the language you want to run it in.
On Windows, find file OmegaT.l4J.ini
in the installation folder and add line -Duser.language=en
(using the language tag you want) at the very bottom of the file.
On the command line execute java -Duser.language=$LANG -jar path/to/OmegaT.jar
(replace $LANG
with the ISO 639-1 code for your desired language)