Here is a short guide to adding a dark mode to grepless. i made a simple browser extension, and its open source! It works on chrome browsers only, as of right now.
1, Go here: https://github.com/diogenesjunior2/grepless-dm
2. Click on the green "Code" button, and then click on "Download ZIP"
3, Extract the zip file
4. Go to chrome://extensions
5. Make sure you're in dev mode
6. Select "Load Unpacked"
7. Choose the unzipped folder
8. Enjoy!