I would like to create web page for my portfolio where a user could select a choice (e.g. radio button) and have that change what is in my CSS Style Sheet so that a particular element changes across the board.
See, what you’re asking for is the CSS to change itself which isn’t possible because CSS isn’t a dynamic language, it’s static like HTML. When HTML changes dynamically it’s not due to the HTML, it’s due to the PHP altering the HTML. Therefore the only way to change the code would be to use scripting or PHP or something to make the dynamic changes as the code is rendered. Which is why you’d need to use scripting - to make the stuff happen as you need it.
I thought it was either Santa (pre-diabetes) or Darwin (looking particularly curious)
Yes it is possible… you would need to set no-caching to the CSS file (as you said) in htaccess (actually perhaps if the code changes it won’t fetch the cached version) and you would use something like this (you could do a style.php file)… http://fwebde.com/css/php-dynamic-css/