I made the following Q&A post about this topic which can be found here if anyone is interested #4024. There were no solutions mentioned so I am going to assume that there is no way of achieving this currently.

This "Ideas" post is making the request to possibly update the translucency settings used in themes. Can think of two possibilities....

-Maybe can update the issue mentioned in post #4024 OR -Maybe can add support for translucent Hexadecimal color codes which might solve the issue. I tried them out but could not get them to work. I am rather confused about these color codes, in general (have not seen anyone mention they work in Geany specifically) some people say they work and some people say they do not, so maybe it depends on if applications support them?

Thank You