As you are more lying with the design skills in the long term, I would suggest you learn HTML+CSS, know a basic foundation of the web design, then learn Javascript, as this script can give you some "mutual activities on your webpage", next, try to learn the new era HTML5+CSS3, and know its power, till here, all front end of a web designing is ready.
Till here, you maybe are able to design a damn beautiful theme, but its typically STATIC, means it got merely no functions, you can't search on site, you can't let others subscribe your site, everything related "ACTIVE", your site just cant do it, because no "people" can do it for your website viewers.
So who can process these functions? The back end language, means PHP, ASP.NET, Ruby on rails, even Python bla bla bla bla, so now if you want to keep learning the design skill, maybe you should pick a language that can let your web pages more active, I would suggest PHP, as it is one of the most popular and got nearly every question covered already on the Internet. Btw, PHP is the back end language of the Wordpress.
So after you truly mastered the PHP, now, HTML5+CSS3+Javascript+PHP, wow, you are near to a good designer already!
As majority CMS users choose Wordpress, so you can keep your learning on this CMS platform, and use your skills to twink it to fit your customers need.
What a long way here, I would assume at least 1 year to learn all of these, maybe 2-3 years.
Cheers!