This is because of the width:50%; you have set for the slide show:
It affects all galleries
You can make that slide show css more specific by adding a template id tag to the body, that way, only albums using the slide show template will have the gallery width restricted.
SOLVED! By commenting out selectively the CSS lines, I isolated that part of the code that seems to affect the Album-set and Album thumbnails. Back in business…for now!
Thank you Rod.