I suspect you mean that there are some dark mode glitches with the topic previews plugin? If I click on the site on your user profile, I do see some hover colors are wrong when in dark mode. Maybe the topic previews plugin needs a small update to better support dark mode (there is a guide for plugins devs), if so, I think it’s worth raising the issue in that plugin’s topic.
We cannot switch themes on-the-fly, themes can have custom javascript code, and it’s not possible to switch the javascript code without reloading the page.