Fix css specificity for content class

[RickCogley]

Css entry for ".content" is conflicting with nanoGallery's class, and causing nanoGallery to zoom photos only into the main content area, rather than full screen as expected. Not specific enough:

.content {
padding: 2rem 3rem 0;
margin-left: 25%;
}

Per Kris-B/nanoGALLERY#77 (comment)

THanks @Kris-B :-)