CartoDB Infowindow color

CartoDB offers 7 presets for Infowindows.

can i change the color of the infowinfow itself, beyond the 7 presets?

e.g. i want a infowindow with a grey header, or a infowindow completely grey with white text.

i think i have to edit e.g. this class: "class="cartodb-popup header blue v2"

but how and where?