The custom theme definitely helped. Custom themes are the main reason I switched from WordPress.com to .org. Feedburner kind of sucks, but it’s pretty much the only free tool out there like it. My main issue is that a good number of people fail to verify their email addresses and there is no way to auto-verify them or send automated reminders to verify. If they lose the verification email or it gets sent to their spam box, the only way to activate them is to delete their account and ask them to resubscribe.
I was considering replacing feedburner with the JetPack subscription form, but my theme from Theme Junkie has a lot of feedburner hooks built in and I don’t have time right now to overhaul that code. Maybe in the future.