I have a bachelor degree in Comp Sci. I know the basics of CSS. I can
change fonts, backgrounds, colors, etc but I can not make complex
layouts.
I am planning to learn Ruby on Rails so I can apply for jobs as a Web
Application Developer. Do I need to worry about CSS? Please note that, I
have no interest in becoming a front end developer and my main interest
is back-end.
You’ll need the basics, probably not much more… but you’d get better money if you can do the whole job, front end, back end, server build, deployment, ops, and all. The more the better. But, I’m still paid pretty well for almost purely back-end work, so don’t sweat it too much.
I would recommend you to at least learn the basics and be able to make a decent pages. These days very good CSS frameworks are available which
make it simple for developers to pick up CSS and UI. As Dave suggested it will be very good to be familiar and have a decent level knowledege of javascript,css,html,devops deployment.
You don’t need to learn much about CSS because for developing a full stack web application in Ruby on Rails you need to focus on so many things to make your application work according to your functionality So focusing on CSS much won’t be worth it according to me.
You should know how to write the CSS and manipulate it; you don't necessarily have to be able to create a fancy web page design, but you should at least know how to implement someone else's fancy web page design. This means knowing enough CSS, HTML, and JavaScript to be able to do that.
Mind that in some companies, rails is considered the frontend.
Everything from rails forward to the browser is frontend, so that
would include CSS and Javascript.
Quite often, from my experience, if you work on a rails project, you
are expected to at least have a good grasp of CSS. Although, if you're
looking for junior positions, the lack of interest in CSS and frontend
browser development might be acceptable.
What I would suggest going forward is working more on your straight
ruby skills, being competent with stdlib and core without gems, as
well as some of the more popular gems. That might help you get and
hold a primarily backend position.