Extract wireframe to a separate theme

[oleq]

ATM, it's hard to develop an application based on CKEditor's UI framework which is to be visually distinctive from Lark. Developers are forced to override Lark's CSS, which produces a tangled mess of specificity problems etc.

So to bypass the problem, a new wireframe theme is to be created, which will define only the basics and let any theme built upon it to define own styling.