I had the "Halo" theme previously installed from Edge Add-Ons and wanted to mess around with some more. I installed a [hideous] "High Contrast" theme from the Chrome Web Store, and the "Halo" theme disappeared from the appearance menu. Installing "Just Black" from Chrome then removed that "High Contrast" theme too.
Is it really only possible to have one externally installed theme, whether from the Edge Add-Ons page or the Chrome Web Store?
Granted, I have not used Chrome itself in ages, so I don't know if that's just a Chromium limitation, or if Edge itself only supports a single custom/paintbrush icon theme. I only switched back to Edge "Stable" because Edge Dev 93.0.910.5 is completely broken for me on YouTube and several other sites.
Anyways, as far as I can remember [and as a comparison], Firefox will actually allow me to "install" multiple themes and not just apply them on the fly. I still have all of my older pre-Proton themes still showing up in my install of Firefox Nightly, albeit with limited functionality due to said Proton re-design.
TL;DR Installing a new extension doesn't automatically uninstall the rest of them, themes should behave the same way lol.