I disabled checking if Firefox is my default browser. It seems to work, that is I'm not being asked if I want to set it as my default browser.
As regards the background color of the Options pages, I have the same version of Firefox as you, but in my case if I start Firefox with ReHIPS disabled, the Options etc. pages are dark. If I enable ReHIPS and start Firefox, then the Options etc. pages are white. I have unchecked Copy User Data.
I don't know if it's relevant, I installed an add-on to Firefox while ReHIPS was disabled. When I started Firefox with ReHIPS enabled, the add-on was missing. I installed it again.
It's as if there are two separate installations of Firefox (at least as far as the settings are concerned), one with ReHIPS disabled and another with ReHIPS enabled.