You are right that removing this would fix the issue, but this code is present in all themes' CSS, so it's probably not the culprit. I'd be hesitant to remove it from all, since that could introduce issues elsewhere on the site. (EDIT: However, I've commented this part out of the code in the WADF and FINCK themes' CSS. If there is any undesired side-effect, we need to find another solution.)
I found something else though: the spoiler tag works by changing an element.style display: from "none" to "block". It does that in the problematic themes too, but there, the user-agent stylesheet overrides the "block" change, so it never takes into full effect. My best guess is that the solution is not in the CSS, but rather the theme's HTML, which probably calls the CSS too early. I'll try to check/change it and see.
On a side note: It's not Christmas time SaimJai!
Heh... living near the equator, I find the sight of snow and winter refreshing! :)