Add better rgb support

[s-cork]

Current failings with _get_rgb:

• we don't support css vars
• we don't support known colors e.g. red
• we have a couple of bugs with misuse of commas in rgb css

Approach in this PR:

• add a hidden dom element to page body
• This hidden dom element is wrapped in a hidden container with an obscure color
• set the style of the hidden element
• use window.getComputedStyle which returns rgb(r, g, b) or rgba(r, g, b, a)
• convert this color to the required space separated values that can then be added to our css vars

Also fixes the tab color is not being applied correctly to the inactive tabs

close #568