Adapting Existing Layout to WordPress

I’ve been in the process of trying to set up a trio of www.ads-software.com blogs for a while now. I know basic coding and already had the layouts designed, but I haven’t been able to find any themes that give me sufficient control of the layout.

For example, every theme reduces the size of my title image or header. I know I could make overrides with CSS, but I’ve found that to be frequently ineffective. The theme administration panels are often glitchy, as well (showing a 404, for example, instead of a preview of my page, or simply not doing what they’re supposed to).

I’ve got the domains, I’ve got the blogs installed on my server, and I’ve got the layout designs, but I’ve been in a holding pattern because I don’t know how to implement my design without relying on a prefab theme set. I even went to a WordPress meetup, but the few folks who showed just told me to conform to the themes.

What’s the best way of moving forward here?