A good overview to change things with css: Getting comfortable with Obsidian CSS
That should allow you to select the background and find the right selector. You can save your changes as a snippet, and they will be applied on top of your theme: Add custom styles - Obsidian Help
And from Discord, Carl the Turtle has this handy recommendation:
Custom CSS lets your completely customize how your Obsidian looks.
To enable custom CSS, go to Settings => Appearance. From there, you can choose among a list of community themes, which appears in settings after you enable the custom CSS plugin.
You can further customize your theme by modifying its file in
.obsidian/themesor by using CSS snippets. To use a snippet, simply save it in a CSS file in<your vault root>/.obsidian/snippets/. We recommend editing the CSS file using something like Visual Studio Code or Sublime Text to properly highlight and format it, as invalid CSS will not work.There’s a wonderful guide on our forum that should get you comfortable with working with CSS, if you’d like to make tweaks yourself: https://forum.obsidian.md/t/getting-comfortable-with-obsidian-css
You can also borrow from existing snippets the community has developed over time here: https://forum.obsidian.md/t/meta-post-common-css-hacks
I believe the solutions posted here should help:
Instead of an image, just specify the color you want.