Well github works the way described by @codebrainz, it chooses between two themes depending on the light/dark setting, but its the only one (of the websites I regularly use) that allows this.
Everyone who works on Geany is a volunteer, they work on what they are interested in and are capable of. They are not directed what to work on. So when/if any specific feature request will be implemented is unknown.
It is unfortunate that this feature is so significant to your use-case that you have to abandon using Geany entirely. No tool is going to be suitable for all and every use-case, I use Vscode for significant C++ projects, but Geany for other less structured uses.