It's been a while since your original question, so perhaps the craftyjs site has changed since then. In any case, I couldn't find a specific element that matched your example CSS selector. So, I'll just provide an example with the site I see today.
Let's use this page as an example: Crafty.settings. Look at those lovely parameter lists! So grey. I hate grey. In fact, I hate parameter lists. Let's pretend they don't exist:
:style -name=ewww craftyjs.com dl {display:none;}
Gone. Ahh.
Now, you may very well ask, why does this work, when your finely crafted example did not? I don't know!
Here's a slightly modified example:
:style -name=ewww craftyjs.com dl {background:#f00;}
Doesn't work! This does, though:
:style -name=ewww craftyjs.com dl {color:#f00;}
Hmmm. We notice that pentadactyl doesn't offer "background" as a completion. Maybe:
:style -name=ewww craftyjs.com dl {background-color:#f00;}
No. Well, perhaps the Firefox developer tools can help us. Inspecting the parameter list box (while sneering silently to ourselves) shows this as the full setting: none repeat scroll 0% 0% rgb(238, 238, 238). Let's change it to blue: background: none repeat scroll 0% 0% rgb(0, 0, 238). Very nice! A comforting retro look. Reset it to gray, so that we can do it with Pentadactyl, like this:
:style -name=ewww craftyjs.com dl {background: none repeat scroll 0% 0% rgb(0, 0, 255)}
Argh. Sorrow. Back to normal. Looks like we found a bug! I like this bug so
much, I gave it a special name: Pentadactly issue #969. Catchy, right?
Update: Aaaand, Peter Wooley FTW! (And now I know more about CSS. Possibly more than I wanted.) Recommended closing the new Pentdactyl issue.