Lack of edit button is due to the permissions issue with user groups at the moment. Sit tight, that will be addressed in short order, by EOD at least.
Posts per page is likely staying at 20 in the interest of load speed. Almost all the modern forum software platforms have a less than 25 setting here. Though we might look into possibly increasing the default of 20 at some point. (We need to see what load times are like once we have some data)
Default theme is intentionally lighter, and intentionally vanilla, since we didn't want to do too much to start with the massive transition from vBulletin to XenForo. A dark theme is on our top 3 for post-launch wishlist items, and has been requested for a looooong time. Shouldn't be too tough, either, since there are a few variants of this theme we can use.
Likes are staying - it's part of core XenForo. You don't have to use 'em, but the kids like 'em.
-JP