I was messing with this page just now.
My custom css looks like this

div.signature {
  display: block !important;
}
span.cuss {color:#888}
div.instructions { font-family: Lucida Handwriting Italic; }

I noticed that the browser is putting in requests to https://djungxnpq2nug.cloudfront.net/css/div.signature { display: block !important
and https://djungxnpq2nug.cloudfront.net/css/}span.cuss {color:#888}div.instructions { font-family: Lucida Handwriting Italic
So is there something screwy going on with the CSS parser?

I don't seem to experience this. Are you sure you're putting your CSS in the correct field? If you examine the network tab of your browser's debugger you can confirm which field(s) have which data in the post request. I'm guessing you're sending CSS code as a URL field... It should also suffice to just inspect the input/textarea element and check its name attribute.

Mine looks like:

I tried posting it again to ensure it wasn't a new "save" bug...

Maybe I am 'special' (my mum says I am) - I tried it on Firefox/windows and Safari/OSX with the same result.
{"name":"610220","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/8\/1\/8163bb1a0a0b46a8c6042508d6b29e36.png","w":2264,"h":1452,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/8\/1\/8163bb1a0a0b46a8c6042508d6b29e36"}

View the HTML. You should just see a call to /1136 on css. I don't see how anything a.cc could be doing would cause those network loads.

You should have quotes around Lucida Handwriting Italic.

Nope.
Depends what you mean by 'should' - it's not required although it is recommended (I tried it both ways)

The request is coming from a file called theme-css.html, the top of which looks like this:

And (if it's relevant) my user css code (from https://www.allegro.cc/css/user.css/1136) looks like this:

/* --------------- REMOTE LINKS --------------- */
div.signature span.url {
  display: none;
}

div.signature {
  display: block !important;
}
span.cuss {color:#888}
div.instructions { font-family: Lucida Handwriting Italic; }

with an extra rule at the top which isn't in the rules shown on the page.

Does anyone have a good, dark theme I could just plunk in there?

Oy, Weems, no! Get out of my thread!

It sounds like Peter is trying to hide something.

Either way, changes only seem to be reflected when linking to an external CSS file, not when filling in the "Custom CSS" text box.

... Never mind. It was a cache issue.

And what is causing this "theme-css.html" to load?

Me. That's the name of the page where I am making my settings.
See attachments. I have a horrible feeling I'm doing something stoopid. Is there anything I can do to be more helpful?

I can see the same thing if I go to https://www.allegro.cc/cc/theme-css with firefox:

And the response:

I'd just assume this is some kind of caching mechanism used by allegro.cc - I have no idea how it works though.

Confirmed. The theme-css page appears to be severely broken.

Right. I initially thought it was every page but I think it's just the CSS editing page that does it.