It's a little bit tricky, but here you go:
Right click anywhere on playlist > settings > script tab (should be opened already > group header sub section
look for these lines:
$font(Ubuntu Condensed,20,)
If you want to use, for instance, smaller Arial bold font, write (Arial,12,bold)

$drawtextex($get(group_title_date),$sub(%el_width%,100),-1,90,,$get(title_emboss),right vcenter)
$drawtextex($get(group_title_date),$sub(%el_width%,100),-2,90,,$get(title_color_alt),right vcenter)
These two draws text (obviously ). After you change your font & size, it should be centered already (depends on font). If it's not, change -1 and -2 to align it, remember that second line is n-1, if your first line is 3(n), second should be 2(n-1).

If you want to change colors, go to Field definition tab and search for text_color_alt and title_emboss. These two are changing color for the whole header. If you want to change color only for dates, you'll have to make some coding!

Also, make sure to check ELPlaylist help on HydrogenAudio, there's a bunch of example scripts and how-to's

Feel free to mod my skins as much as you want, if you still need help, just send me a note!