Looking for more articles on creating custom themes

I’ve been hand-coding websites for 10 years but now I’m strongly considering moving most of my projects to WordPress as a full CMS. [related post]

I know HTML and CSS very well and some php but not a ton. I’ve read articles on this site about creating templates and using “the loop” but I still don’t fully understand all the specialized WordPress code or the more advanced php that goes around it. I know I can design graphics to simply fit existing templates, but what do I do when I’m suggesting to move my client’s website to the WordPress platform but I need it to look 95% like their old hand-coded website? Like just for example, maybe I need their website to be exactly a certain width, or a certain padding from the top, or not have a border around the content area, or a totally non-wordpress navigation menu, or whatever small things add up to make their website look like it does. Like I said, I know HTML and CSS but I’m not fully grasping how WordPress puts it all together. Can anybody suggest good 3rd-party articles, not on this WordPress site, that may better help me in understanding how best to ‘translate’ or ‘repurpose’ my existing design templates (headers, footers, menus, etc) into usable working WordPress themes? Thanks.