Well, I can't be specific here. Tbh, your theme's just perfect if subtlety is concerned. It's really good on the eyes and goes nicely with your profile image. Its just that...some people may or may not like the subtleness of your profile. Dunno... Argh! I'm confused now! D:
It does make some difference. But you know, profile pics are just there to make profiles look prettier. If you can make it look prettier with a profile pic, then go for it. If not, then just scratch the profile pic idea off. xD;
It's not like you HAVE to change your profile with your theme. A theme needn't match one's profile. But yeah, you have to make your profile picture match your customizations.