Editing the properties of one sidebar without affecting the others

In styles.css I have several sidebars with names such as: sidebar1, sidebar, sidebar-left, sidebar-right…

Their properties (such as padding, margin, line height, font and so on) are in a css section called:
ul.sidebar_list li ul, ul.sidebar_list li div

I wanted to put a jwplayer in the first sidebar. I had to get rid of all the inner padding, so the player would fit in it nicely, with no white space around it. It worked great, however, the changes affected the other sidebars, and I didn’t want that.

How to solve this? How to edit the first sidebar independently of the others?

I hoped that I can solve this by using a plugin like “custom sidebars” but it doesn’t work for me (it doesn’t appear)

Thanks!