I essentially retained the default L&F of phpBB3, and propagated it to the site itself to give a more integrated experience. In many ways, what I've done isn't that clever really.
I'm currently working on a gallery for it - if you follow the gallery links you'll get taken to a "Coming soon" page but of course I've coded a backdoor to enable me to develop this. When done, I'll either take the backdoor out or render it ineffective so I can re-instate it and use it again for debugging purposes.
For a personal blog, you probably don't need much beyond HTML and CSS, if that really.
I think it helps to have a project in mind before you start writing any code, and to write some kind of functional specification first as well. It gives you a focus and something to work towards. In the case of the gallery on the model flying site, I wrote a loose specification before starting any work as it gave me a starting point and a direction.