This bug has been around since they introduced the feature and it’s still there… if you right-click a chat window and select “Disable Light Background” it will be reset when you login again. You have to do this every single time you launch the game.
This is with the official Windows and Mac launchers, untested with the browser version.
It doesn’t do that for me but every time I add a chat window to a stack it turns the light background on and I have to manually turn it off again. Since the bug report about the faulty char sheet they manged to remove the light background transparency from my char sheet but there is still no way to turn it on or off for the char sheet. The System map (F9) also has no way to do that.
It’s just another buggy, incomplete UI feature that no one asked for and that CCP abandoned right after release.