It worked for me after including the CSS styles at the bottom of the file, because there were some rules that overwritten this changes.
/* Add these rules to the top of geany.css file to make dark */ * { background-color: rgb(25, 25, 25); border-color: rgb(25, 25, 25); color: rgb(150, 150, 150); } button, button *, *:hover, *:hover>* { background-color: rgb(150, 150, 150); color: rgb(25, 25, 25); }