Linda! Bless you, bless you! That indeed solved the problem. I'd looked at that section of code earlier -- but I hadn't been thinking that the 10 pixels referenced there (for the top AND bottom) was creating the 20-pixel space problem. I'm very rusty with my CSS, so it didn't jump out at me. Thanks so much for posting the solution, which is bound to help other folks, too.
I've also been encountering some other issues that have made it difficult to determine the cause of various problems. This is my first time to use WordPress, and right now I'm creating my blog/website (using Jai's great Rhea template) on a local server on my Mac, working in Safari. After experiencing a number of quirky things, I finally found references online where folks have been having problems losing formatting in WordPress' Visual Editor when using Safari. So I moved over to Firefox, where the Visual Editor is working just great. However, some changes are taking "forever" to take effect (which was also happening in Safari). I'm not sure whether this is a browser issue, or is the result of working on a local server. Nevertheless, it's driving me crazy at times, because some changes are taking a while to show up. So at times I'm wondering if I've actually made the correct change, as I wait for the change to hopefully take effect. Have you (or anyone else reading this post) experienced this type of sluggishness with changes taking effect in WordPress?
Anyway, thanks again for your help with the spacing issue -- much appreciated.