cubicweb #893244 CSS: ``h3`` and ``h4`` are too similar [validation pending]

h3 and h4 are too similar, so a blog with h4 subsections for example is hard to read, since its structure is not visible.

prioritynormal
typebug
done in3.14.7
load0.300
load left0.000
closed by#20e38436395f [css] change `h3` and `h4` styles(closes #893244)