Switch to Solarized color palette

[mizdebsk]

Currently it's hard to read Validator HTML reports due to poor selection of colors and in some cases low contrast (eg. yellow text on white background).
Instead of inventing a new color scheme, I propose to use Solarized: a set of carefully selected colors designed for use in applications like Validator. Solarized improves readability and reduces eye fatigue.
I understand that this is a subjective matter (de gustibus non est disputandum), so feel free to ignore/close this PR.

[mkoncek]

The idea sounds good.
I have to check how the results look like myself.

[mizdebsk]

Example outputs:

• solarized: https://mizdebsk.fedorapeople.org/jpv-demo/solarized/html
• current: https://mizdebsk.fedorapeople.org/jpv-demo/current/html

[mizdebsk]

Also added the code that was used to generate the example outputs.

[mkoncek]

I think it makes sense to use a generally accepted color scheme.
We can put it to test use now.