CFProject Questions & Answers

First you create a CSS class for every theme you need.
Then you use javascript to change the class of the elements you want to change the colour of depending on the selection of the combo box like this simple example:

What this does is change the foreground and background colours of the whole HTML page depending on what the user selects from the drop down box.
So, each CSS class has a different colour theme.
The drop down box has the class name of each theme as the value of an option
Blue has the same value as the class name blue in the CSS.
When the user selects something, the classname of the HTML body (document.body.classname) is set to the value of the option value and the class automatically changes so the theme changes.