Your styling should be done using CSS wherever possible. Have different classes setup according to your needs, then add or remove classes when absolutely necessary with JS.
One thing to keep in mind is that changing styling via JS is a one-time change. Elements added dynamically VIA Ajax won't inherit the styling changes automatically. Another good reason to stick with CSS.
See this post for additional confirmation Should I load responsive design via JS or CSS
As suggested in the link
Putting everything regarding styles in the CSS files is the best
practice.
HTML => Structure
CSS => Styles
JS => Logic
