where to start with theme development

[jeffree7]

I'm trying to modify the classic them a little more to my liking, so far I'm only able to add .css to the custom.css file in /themes/my_theme/assets/css/custom.css

One of the big things that I'm having difficulty with is making changes to the imageslider, I'm not sure where or how to do this. I want to make the imageslider the full width of the screen but I'm not able to with my css knowledge or I'm looking at the problem wrong. Since the imageslider is a module should I be looking in the .tpl file for that module? I watched a video of someone doing just this but with version 1.6 and they were able to change the width using the admin interface for the module in the Back office. 

Also, I'm looking for just a push or point in the right direction. I'm not sure if I should look into smarty templates or I need to figure it out with .css

 

 

Thanks