Emacs background colour does not switch to black after picking a theme

I have a really odd problem, which I don't see to be able to solve. This is is Emacs 28.2 (on AWS WorkSpace Linux 2, but probably not the issue). My default background in Emacs in the terminal is black. Great When I do \`customize-themes\` and pick for example wheatgrass, my default background isn't exactly black. I do \`customize-face\` and set background to black, but it does not become black. It looks like the theme continues to override my customisation. How can I enforce the black background? This is in terminal emacs (nox).

BTW, out of the box the background is dark. It's only when I pick a theme that I can't get black back.

You need to disable the old theme when you pick a new one, otherwise they can become mixed. M-x disable-theme.

I'm starting from a position of no themes: \`customize-themes\` shows nothing ticked. I pick a theme, then the background changes from black to something not exactly black. Yes, disable-theme reverts me back to the black background, but disables the theme as well.

So your question is actually "how do I override the background for a theme?". `set-background-color`?

I can call that, and give it "black", but it won't be black. Similar to all kinds of other ways like \`customize-face\` default. It's only on this setup where I suffer this issue. I just don't understand it.