You don't need !important when modifying CSS with jQuery since it modifies the style attribute on the elements in the DOM directly. !important is only needed in stylesheets to disallow a particular style rule from being overridden at a lower level. Modifying style directly is the lowest level you can go, so !important has no meaning.
If you really need to override css that has !important rules in it, for instance, in a case I ran into recently, overriding a wordpress theme required !important scss rules to break the theme, but since I was transpiling my code with webpack and (I assume this is why --)my css came along in the chain after the transpiled javascript, you can add a separate class rule in your stylesheet that overrides the first !important rule in the cascade, and toggle the heavier-weighted class rather than adjusting css dynamically. Just a thought.