Derfeluk,
Glad you put the tip to good use; great site, indeed well integrated!
It should not break validation; inline CSS is legal. Something like this:
<p><span style="font-size: large">I highly recommend it</span> to anyone needing a professional look for their picture collection display.</p>
You can also link to your own stylesheet residing somewhere on the Web (so you can reference an absolute URL). I think that is illegal, attaching a style sheet from within the <body> tag (rather than in the <head> tag) but it worked nevertheless in my tests.
You may also be able to edit a Shozam Theme CSS outside the program though this is not supported. Save the theme under a new name and then edit the corresponding CSS in the [My Documents/My Shozam Themes] folder. Back-up first and test to ensure the theme does not get "corrupted" (does not get recognized by Shozam). You should then be able to apply that externally edited theme from within Shozam and not have Shozam write over your edits.
NOTE FOR OTHER READERS: Suggestions above are for pro Web designers with understanding of CSS editing.