Sorry didn’t see your question till now. The file I edited was located here:
C:\Program Files (x86)\UiPath\Orchestrator\dist\default.theme.css
Just make sure to take a backup before editing as some changes may break Orchestrator if you’re not careful.
The default colour is #1976d2 which you can “Find and replace” to make it your own.
Also, you’ll need to Minify your CSS for it to work properly. I was getting “Invalid character” errors in Orchestrator until I did this.