If you try to edit a page like Template:Editnoties/Page/MediaWiki:Common.css it incorrectly shows you the CodeEditor interface, and it tries to comprehend your input as CSS code, not as wikicode. It even prevents you from saving the page if your wikicode appears like invalid CSS code (which is often the case) with the error message " Missing selector list at line 1 character 1."
This happened to me on fawiki just now.
I believe this is a side effect of TemplateStyles which converted all subpages in the template namespace with the .css suffix into the CSS content-model.
@Huji I think you should ask an administrator to change the content-model of the page.
There's nothing more we can sensibly do here beyond the workaround mentioned in T217776#5007529, as it would be prohibitive to try to configure lists of local exceptions for every wiki.
For the record, point your administrator to Special:ChangeContentModel, or to the "Page Information"[1] link in the sidebar (which takes you to a ?action=info URL) where you can find a "change"[2] link on the row for "Page content model"[3].
[1]: I18n message: pageinfo-toolboxlink
[2]: I18n message: pageinfo-content-model-change
[3]: I18n message: pageinfo-content-model