Obox recently released DevKit as a CSS and Javascript add-on for any theme. What they’ve released as DevKit on CodeCanyon is what has been powering the user-added code features for their builder theme, Layers. Layers was released early this year with a lot of good feedback (including from me).
DevKit looks really slick, if you must edit CSS and Javascript from the browser. It does so using a Customizer like layout, just like the rest of Layers. Of course, I’m averse to people writing CSS and Javascript directly in their CMS, but sometimes it is the most convenient or potentially the only way at all to get something done. This is the same justification I offer for the Jetpack CSS module. And in these cases, DevKit seems really nice.
I’d actually like to see Jetpack move their CSS editor to the customizer. It makes total sense.