Can't change the body background image for a specific page

[unreal ravine]

I have a page called "Template:DaysUnionStyle.css" with a bit of CSS code in it that is added to another page via {{#css:Template:DaysUnionStyle.css}} that gives the page a style different from other pages on the wiki, however, all code in the screenshot works, except for when "background-image" is added below "body", and I am not sure why this line of code does not work.
How would I fix this?
Page:
https://mythcommunity.miraheze.org/wiki/Template:DaysUnionStyle.css
Page that the template is used in:
https://mythcommunity.miraheze.org/wiki/Great_Day

[steady hazel]

Perhaps that template uses TemplateStyles?
You might try enabling the TemplateStylesExtender

[unreal ravine]

How would I do that?

[steady hazel]

Special:ManageWiki/extensions

[unreal ravine]

Thank You

[steady hazel]

Sorry for the confusion if you are not using it.
But it is useful and I urge you to try this.
https://www.mediawiki.org/wiki/Extension:TemplateStyles

[unreal ravine]

Yes I was just looking at that, but thank you regardless lol

[open thistle]

Stuff in TemplateStyles is sanitized by default, so no styling rules outside of .mw-parser-output will get applied for security reasons. You would need to change Common.css instead. The problem, of course, is that stuff in Common.css gets applied to everything. If you only want it to apply to specific pages, you may need to write javascript in Common.js.

[unreal ravine]

Are you saying I am unable to change the body background for a certain page with TemplateStyles?

[open thistle]

Yes. You can observe this in the browser's dev tools. All selectors are prepended with .mw-parser-output.

[median swallow]

How do we know which stylng rules are outside of .mw-parser-output?