ok, good

Code
#island_calendar_table .tdheader {
  background-color:#2153AA;
}

will change only the tdheader class inside the island_calendar_table (id) for example

you'd need to make sure that css is included after the original definitions. (my guess is style.css gets loaded last, so put that in the bottom of that file)

then the cascading part of Css take effect smile

to go further, say we wanted to restyle the link (A tag) for "october"

Code
#island_calendar_table .tdheader a:link {
  color:#FFFFFF;
}

would make it White.. and so on..

the selectors themselves cascade wink

finally, make the alt-1 class inside the div have a lighter background

Code
#island_calendar_table .alt-1 {
background-color:#F3F7FB;
}

end result is: [Linked Image from sirdude.com]

and it doesn't overwrite any of the existing .tdheader, .alt-1 rules, since i make these ONLY apply INSIDE of that div

ps, i also recommend a Firefox plugin (Stylish) that gives you even more control over the Site CSS (for testing)..

Colorzilla is a must too..

Matter of fact, i use stylish for THIS site, to restyle certain things from the supplied ubbcentral default style.

2c