Menu Icons barely visible with Light Background Image

Nextcloud version 25.0.2 on OpenSuse Leap 15.4 php8 (8.0.25-150400.4.17.1) apache2 (2.4.51-150400.6.3.1)

I created a slick background image for my web users with a nice bright top. But then my icons are hardly visible (off white on a cloudy sky).
Q1) Am I right that there is no admin settable background color / shade for the menu?
Q2) Am I right that there is no admin settable foreground color for the top menu icons?

So I tried to fix the background color issue on tip and added a darker area on top of my image to go behind the icons. This works fine when my browser is not too wide. Then the image is scaled between top and bottom, cropping left and right (preserving the aspect ratio).
But when I widen my browser window, the image is scaled between the left and right side, cropping top and bottom (preserving aspect ratio).
Q3) There is no other option than to “center” the background image? In my case it would be good if I could fix it to the top or not preserve aspect ratio. Or better still: create a settable background color.

(left full screen browser, right narrowed down)
or smaller window login screen: