Improve color/style of h4 headings

[Andreas Motl]

Hi there,

@proddata suggested at #248 (comment):

I think making the h4-headings a bit darker would improve the readability quite a bit.

With kind regards,
Andreas.

[Andreas Motl]

@belringer responded at #248 (comment):

I suggest color #000000 and additionally font-weight: bold;

[Andreas Motl]

I believe this has been implemented by 26a25ef already.