This repository is a compilation of themes and stuff for QB64. Pretty straightfoward.
To install themes, go to your config.ini file, found in the internal directory. In the section "IDE COLOR SCHEMES", append it with Scheme<number>$=(QBT file contents), for example:
[IDE COLOR SCHEMES]
Instructions1="Create custom color schemes in the IDE (Options->IDE Colors)."
Instructions2="Custom color schemes will be stored in this section."
Scheme1$=Night Coder|226226226039157000245128177137137137177196000196118000000000000049049049098098098
Scheme2$=High Contrast Light|000000000000000108245000000137000039000088000098137088255255255245255088000147177
Scheme3$=High Contrast Dark|255255255000157255245049078206177000000255000255078245000000000020020029000147147
...If you do not have this section, don't worry, it's completely normal. Simply copy and paste template.ini under the "IDE DISPLAY SETTINGS" category of your config.ini file.