When you click on the CODE tab for the Overview page there is a backgroundImage option. I imagine there is also a backgroundColor option as well. Though the color choices are limited to the F7 set of colors. See oh-equipment-card - Equipment Class Card | openHAB for a list of the colors.
No, you can’t eliminate the tab. It’s a tabbed Page and there is no way to change that.
You can do it per device in the Help & About page > “Standard home page background color”.
You can change the navbar to a fixed one with “Simple navigation bar on home page”.
There are no ways to configure this for all devices yet.
That’s true, but if you edit the page to change “Display model cards” and check “Administrators” only, regular users (including those who are anonymous) won’t see the bottom tab bar, only admins will.