The first thing you should be doing is getting better at HTML and CSS. The Codeacademy course is a good intro to the subject, but you need to write more code to truly internalize it.
The next step would be to look into learning more advanced CSS. Look into preprocessors, such as Sass, Less or Stylus. My personal recommendation would be Sass with the SCSS variety (http://sass-lang.com/). Sass over the alternatives due to the sheer amount of open source projects using Sass over others, Bootstrap is also in the process of changing to Sass from Less. There are several good resources online for getting into Sass. One word of advice: Never EVER nest selectors (at least until you understand why you shouldn't overqualify your selectors).
Read up on and learn about Object Oriented CSS, SMACSS, BEM, ITCSS and finally start using BEMIT naming conventions (https://csswizardry.com/2013/01/mindbemding-getting-your-head-round-bem-syntax/). CSSWizardry is a very good authority on writing good large scale CSS.