Hi.
Sorry about that. We edited the CSS a bit to improve compatibility for plugins that add custom content inside posts, and we overdid it a bit, leaving the excerpts out of the configurable options.
We’ve already fixed this and submitted the update. You can grab the fixed mantra/admin/custom-styles.php file right now from the 2.0.5 archive and replace the existing one.
Also add these lines to the custom CSS field:
.entry-content, .entry-content p, .entry-summary, .entry-summary p {
font-size:15px; line-height:1.7em; }
.entry-content p, .entry-summary p { margin-bottom: 10px; }