Feature Request: Allow custom CSS

[rubasace]

It would be very useful if the plugin added all CSS still respecting custom CSS by the user. This might allow knowledgable users to fine-tune the skins at their will.

One way could be somehow flagging the beginning and end of the skin CSS code and only replacing that part when setting the skin.

Alternatively, an extra input where users can put their "extra" CSS could also be useful.

[DexLuther]

Alternatively, instead of applying a skin directly have the option to show the CSS instead. That way experienced users can merge it with their own and/or tweak even the built-in skins further. Especially to make them compatible with mobile, which they don't seem to be at the moment.