External stylesheets and custom controls

[BJF]

I may be missing something obvious, but I need a point in the right
direction.
I want to be able to create an external stylesheet with style attributes
for a custom control.

What I Know::

I know that if I wanted to apply style elements to, say, a <p> html
element, then I'd use this CSS syntax:
p
{ property: value; anotherproperty: anothervalue}

I know that if I wanted to apply style elemts to all <p> html elements
with a class of 'thisclass', I'd use this CSS syntax:
p#thisclass
{ property: value; anotherproperty: anothervalue}

What I Don't Know:
If I create my own custom class 'myclass' and add it to my page with
this HTML:
<cc:myclass> ... </cc:myclass>
What should my selector in the external style sheet look like?

I tried
cc:myclass
{ property: value; anotherproperty: anothervalue}

and it didn't work.

If I want to use the style sheet to set attributes for all of my custom
controls with a style class of 'thisclass', assuming I've remembered to set
the CssClass property of my control's instance to 'thisclass', what selector
should I use? I tried this and it didn't work:
cc:myclass#thisclass
{ property: value; anotherproperty: anothervalue}

Any ideas? I'm just not finding this documented anywhere.

Thanks in advance.

Ben