Thanks for comments, guys!Dave: As the meattr of fact, it did bug me, and it drove me nuts. Even if I upload an image which is *IDENTICAL* to original WordPress background for this theme (with nice corners etc.) it simply sinks a little bit down, revealing two more rounded corners, and then it looks ulgy. The problem is in CSS stylesheet file, which contains two header definitions as soon as you upload your custom header, and that’s something that I can’t do anything about. WordPress doesn’t let me change the CSS, unless I pay $$$. And I’m not giving them a penny on general principles. I’m rather gonna pay some professional hosting provider giving me possibility to upload my own theme, rather than just customize an ugly default one. And in long run, I hope I will do just that, move this blog to its own domain, but its a long shot.
Needs Rounded Corners 
Thanks for comments, guys!Dave: As the meattr of fact, it did bug me, and it drove me nuts. Even if I upload an image which is *IDENTICAL* to original WordPress background for this theme (with nice corners etc.) it simply sinks a little bit down, revealing two more rounded corners, and then it looks ulgy. The problem is in CSS stylesheet file, which contains two header definitions as soon as you upload your custom header, and that’s something that I can’t do anything about. WordPress doesn’t let me change the CSS, unless I pay $$$. And I’m not giving them a penny on general principles. I’m rather gonna pay some professional hosting provider giving me possibility to upload my own theme, rather than just customize an ugly default one. And in long run, I hope I will do just that, move this blog to its own domain, but its a long shot.