Hi there,
Teaching Rhino in German this week (ouch, it’s way too rusty, makes me sad)… My normal layout setting is a white background. So I opened my carefully prepared (in English Rhino) file that has a bunch of layouts in it - and in Rhino in German the layouts have a gray background (default setting I believe)…
Unfortunately, there are a bunch of guidelines in my layouts that are in light gray, so they are invisible against a gray background. No problem I thought, just have to go into Options>Appearance>Colors and change the layout background setting… But nope. I can set it to white, but the layouts stay gray… 
Known bug? Something I missed?
English Rhino appearance for reference:
I just read Layout and Hintergrund and assume it was layout background… Thanks!