OK, done.
The problem was not with CSS, but with the image. (and yes, I told you about that 2 days ago)
If you used small image (310x163px) that was mechanically resized by the browser - you didn't see any details, only solid blurred dark blue (high-school physics classes).
I used much bigger image (and similar design): /images/image_interested.webp
and now that background is visible on ALL pages where you have SPPB.
All my CSS is here: templates/investa/css/custom.css file
I found orginal image that you used (with watermark) and uploaded it here: /images/image-interested.webp
So if you don't believe me (you've right), you can always change to that image and compare view.
In summary:
- For the desktop/laptop background, you should use an image that is big enough.
- We all make "stupid" mistakes from time to time ;]
- For small Mobile screen I would use solid color, not image that people even will not see, or use different image, vertical shape, for example, 680 x 2800px.