https://www.hex-rays.com/products/decompiler/manual/config.shtml here is how to configure IDA, for new projects it work, but not for existing one.
Mostly I am interested in changing MARK_BGCOLOR.
I have already huge database, and I found out that the default color is not very readable on my screen, so I want to change it, originally I think it do not work at all, but then I create new database on small exe to speedup the loading, and it worked, after bit of investigation I found out that this config is used only when I click new in welcome screen, but not for already existing databases.
Is there any way to update that value in existing idb?
It is probably the only color that can not be overwritten by color schema.
The issue can be resolved by using latest IDA (7.3 at the time of writing). Changing the config works for existing databases now. It is as easy as that if you can afford the update, if you can not, there will probably never be a solution :(