create a function in css?

No, there are no functions in css.

There are two solutions that I can think of:

Minimise the length of the class names, such as class="btl bt btr br bbr bb bbl bl" which will minimise your typing.

Use a preprocessor such as PPWizard. In PPWizard you could set up a deinition:

#define mycurvedbox curvedboxtopleft curvedboxtop curvedboxtopright curvedboxright curvedboxbottomright curvedboxbottom curvedboxbottomleft curvedboxleft

Then instead of writing:

class="curvedboxtopleft curvedboxtop curvedboxtopright curvedboxright curvedboxbottomright curvedboxbottom curvedboxbottomleft curvedboxleft"

You would just write:

class="<$mycurvedbox>"

For more on PPWizard see http://www.html-tags-guide.com/html-software.html

Rather than use 8 separate classes, it may be easier to use the CSS3 border image properties. See: http://www.w3.org/TR/2002/WD-css3-border-20021107/...

However if you do, you should check that it works in all of the browsers that your visitors are likely to use. For that you may want to use http://browsershots.org/

Alternatively you could use the more usual approach of four areas or divs to cover a corner and side each with a background image that is longer than any conceivable top/bottom or side edge.

To call multiple classes at once separate them by commas as demonstrated below.

<style type="text/css">

.class1, .class2, .class3 {color:#FFF;}

</style>

You can reference multiple classes for an element by separating them with a space as demonstrated below

<div class="class1 class2 class3">Content</div>

CSS is not a programming language in the traditional sense, so the term "function" does not apply here and so I'm not sure if this is the answer you're looking for.