Hi, nice Idea and could boost productivity, but it does have some flaws and could be improved, like

• load the ui preferences from user or browser preferences (like dark page option from opera gx users)
• popup could be a little bit more wider to avoid vertical scrolling on wider project descriptions (could be added to options to let the user decide how large this window could be)
• cannot access private repos even when I am logged in into github - As I understand you are doing API calls with "some" user / anonymous user? The user should be able to login to the own user to allow the extension to access the users repos.

I will give it a try on long term.