You can enable editor dark theme by going to settings and setting dark as the editor theme.

https://storage.googleapis.com/replit/images/1548117951238_b01a14be16734333b9019ea9e6e3b6dc.pn

https://storage.googleapis.com/replit/images/1548118012958_005f076ff600aa332479cf1ae039e315.pn

If you'd like a dark theme for the entire website, you can go to https://darktheme.tk and follow the instructions there.

Please upvote my comment if it was helpful or useful to you. Thanks!

1. Click on settings
   https://storage.googleapis.com/replit/images/1548117917530_ee89577c920be873487eea3bc4bbe845.pn
2. Select 'dark' under theme
   https://storage.googleapis.com/replit/images/1548117928978_f9ce7397aae3216c45abbd0ef1a5d43b.pn