hi Stitch 

Let's  see if I can explain something to you.    The css (the stylesheet)  defines the look of the theme.   When using the styleswapper codes that you have tried, they do not, as you say  "cover up"  the default theme.    That is not possible.       
 
What happens is when you choose a style, you are choosing to use THAT stylesheet.   
That stylesheet (the css) contains the information for the images, fonts, etc for the forum.    It does not cover up the default stylesheet.. it replaces the default stylesheet and uses the images/fonts/etc defined in the css.   

The styleswappers that Moose made do need to have the default stylesheet set as the default  and DO flash  - and he has said that he cannot change that.   

The one that I created (for use in the portal, or the helious one) does not flash and you do not have to use the default style to use it.   You can choose any style you wish as the default style. 

It "would" be cool if SMF could find a way whereby members could choose the style they wish to use,  I do agree.    But I did want to clear that up for you.

It is on my planned feature list.