A Dark Mode for Roll20 is a popular Feature Request, but it haven't yet been implemented.

Roll20 have started with creating a Dark Mode for the Help Center.

• Dark Mode Suggestion Discussion^(Forum) - You can go and vote & comment on the suggestion.

Workaround

Using the Stylus- browser plugin, you can try using one of the community-made attempts at making a Dark Mode.

Check Show off Your Style 3: Third Time is the Charm^(Forum) for more info and good tricks others have created.