There's a bunch of really nice colour schemes available from the geany-themes project. Consult the README* file in each release/repository for instructions. In Geany, check the Help menu and About item to check the version you need to download.
Download one of these:
Download one of these:
Base16 provides carefully chosen syntax highlighting and a default set of sixteen colors suitable for a wide range of applications. The template for Geany gives eleven, dark, easy to read schemes: