No, not really. While Roundcube is skin-able, doing a new skin well (so that everything works, including popular plugins) takes some effort. And since the RC developers are an active bunch, keeping a skin working properly from version to version is a lot of work, and takes a lot of time. Frequently, by the time someone gets a skin completely de-bugged and working well in one version of RC, the next release is almost ready and the skin developer needs to re-work it all over again. So there have been many nice skins over the past couple of years that have just been left behind as RC continues to mature. The lack of skin selection is (IMO) directly related to the speed of RC development.
And given a choice between more skins or better webmail, I'll go with better webmail every time.