Different background colors for several different pages

Hi everyone - my team and I are in the process of building a new website for our online magazine, and we're a little stuck. Since we're issue-based, and all the issues are themed, we're creating a separate blog page for each issue. We're hoping to give each issue, along with all of its articles, a unique background color. I've done a lot of googling and custom CSSing, and I've created a color theme with a transparent background so that the custom color will show through, but nothing seems to be working. Any help would be greatly appreciated! The site isn't published yet, but I'm happy to provide screenshots/explain more of what I've tried already.

Link to the site and just share a password.

Also, at a guess, try creating a code block in each blog post (anywhere is fine, I'd probably go for the end for simplicity) and paste this into it, changing the two values for text and background.

<style>
/*Squareskills - Change Blog Post Colours*/
article .entry {
  background: YOURBACKGROUNDCOLOR;
  color: YOURTEXTCOLOUR;
}
article .entry :is(h1,h2,h3,h4) {
  color: YOURHEADINGTEXTCOLOUR!important;
}
</style>

Again, at a guess, I have a feeling headings need separate targeting but if you want uniform colours you can likely just use !important in the first bit of CSS and skip the bottom half.