Welcome to the forums Max. Were you running Windows 10 and 8.1 via Dual boot, or had you upgraded from 8.1 > 10 and then removed the old 8.1 data?
Could you please try right clicking somewhere empty on the desktop and select
Personalise. Then, are you able to change the background colour to something more satisfactory by choosing an image/colour from the list shown? If you want to revert it back to the default Windows 10 appearance, click
Themes from the sidebar and choose the "Windows" theme:
If that doesn't help, can you please attach a screenshot to your reply, which may demonstrate the problem to us
. You can use the "snipping tool" (search for this on the start menu) to take a screenshot.