I want to know how to customise the font, colour, text size, etc, of the game window title. My current workaround is to make the game borderless and implement a custom top bar but I would prefer to be able to customise the title without doing this.
Thanks in advance.
That part of the application depends on the Window Manager, which is par of the OS, so applications should not have access to it. The usual way of making custom top bar is using borderless, as you are using, so nothing to add there.
If you want to emulate the look of the OS, you can pull some variables from the system (godot already does some of them for you), but it gets complicated fast.
Bottom line: you can’t do that.
As mentioned above, you need to draw your own window border to achieve this (also known as client-side decoration). The OS’ theme settings define how window borders look.