.postbit_buttons is a wrapper class and .postbit_[buttonname] are individual classes.
.postbit_buttons > a:link, .postbit_buttons > a:hover, .postbit_buttons > a:visited, .postbit_buttons > a:active is the main selector for changing background etc.
EDIT: nvm, the answer below is correct for the specific theme (didn't notice link). Moving to Theme Support.
edit: delayed response
megabg.css (edit through admin panel & edit in advanced mode)
.pbbutton a:link, .pbbutton a:visited {
background: __________________
to inspect element just right click on the element you want
.blockquote
.blockquote cite
.blockquote cite span
.blockquote cite span.highlight
.trow_reputation_positive
.trow_reputation_negative