First of all differ between the stuff. WP is not CSS and CSS is not WP. But WP themes make use of CSS. So if you learn about CSS you're basically not doing wrong (good thing with CSS is, it is common between all websites). The other part is HTML. And then you have PHP.
So these are three computer languages that are used with each other:
- a programming language: PHP
- a structural language: HTML
- a domain specific language: CSS
Then you have a multi-layer network architecture: PHP is executed on the server and HTML and CSS is provided by the server but read / processed by the Browser on the client computer.
So even if themes might look simple, with WP this all gets interchangeable mixed and you must yourself put the stuff apart again.
PHP is in "Theme" (PHP) Files, HTML is generated (and as well is) inside those Theme files, CSS is in the CSS file.
So basically you need to decide yourself what you want to learn. Most resources differ between PHP and HTML/CSS.
Based on your question I can only suggest to you to learn CSS. It is very powerful after you have fully understood the Cascade. You need to know HTML forehand (not PHP, just HTML). In the end CSS is absolutely important for the visual representation of a website, it's a power tool to decorate any website. So you can use that with everything, even wordpress themes ;)
Go for it. If you haven't understood until now after 4 years, spend yourself some education. I suggest some book like http://oreilly.com/catalog/9780596101978 which is perfect for self-education (next to practicing).
CSS itself is very well deined by the W3C as well as HTML is. The definition there is not very user-friendly but it's correct so with some will you can learn a lot there as well. w3schools is nice made as well (and that site somehow makes it over the years, looks at least some brain is behind it). HTML/CSS, that is the WWW, grab your source :D
For PHP there is php.net. For wordpress, well, it's always changing and consumes resources of your personal career, so do not focus too much on to it. Its well kept developed anyway so, just look for whats good for you, do not focus on wordpress too much.