As I am sure you already know Joomla is an open source content management system for websites. Since it is open source, this means that anyone can write an extension of the core components that come with the initial software installation and the new extension will be able to work with and to extend the functionality of the original core components.
However, to write various extensions of the core software components, one must have a little knowledge of HTML codes and CSS codes. Without this knowledge, writing the extensions can prove to be very difficult. The extensions have to be written just right, so that the core components will work with them.
Cascading Style Sheets (CSS) help give the website its fonts, headings, borders and the alike. You will not be able to change the fonts, unless you change the CSS coding. Style Sheet codes help to give the little touches to the overall feel of the website.
Even if you can write in HTML coding, that coding will have to be converted over to CSS as the Joomla software requires that you have both in order for the program to function properly. Being an expert in HTML will not help with CSS because it is essentially two different "languages". In other words, you can structure your style sheets to HTML standards without compromising the look and feel of the website.
CSS templates help to provide smaller file sizes and faster browser rendering. Search engines love websites who have smaller file sizes. This is the advantage of the CSS templates compared to the HTML templates.
I know, all of this can be confusing. Do not be scared by all of this, unless you have a background working with style sheets or HTML, this can prove to be extremely confusing. Believe it or not, these programs style sheet an even be confusing for the professionals as well; however, if you will search the internet you will find several different tutorials available that can help you to understand more about this subject.
There are people out there who can do this or we would not have all of the various extensions to the Joomla core program that we have today. So, if you find that you do need help understanding all of this mumbo jumbo, you sure can find someone that can help you with it.
One of the best pieces of advice that I can offer you, is to visit the "help desk" for this open source software program. They will be able to answer any questions that you may have. TeachmeJoomla is another website that I have found that can be very helpful to the Joomla programmers or non-programmers that want to try. Another extremely helpful website is that docs joomla website. You can find Joomla documentation on anything and everything that you want to know.
This software and all of the codes involved can be extremely confusing for the majority of consumers out there; however, do not be afraid to ask for help. You can research it as well, as you will find many websites that are dedicated to this topic.