How familiar are you with CSS? The right sidebar is composed of multiple blocks, and the blocks that you are probably wanting to style are the
#rightSidebar. These are already styled in the vanilla theme via
#sidebar div.block, and your override will need to be at least as specific as that.
If CSS is a new topic, W3Schools has a fantastic CSS tutorial. You may also find it helpful to explore the existing styling via your browser's inspect tool (for example in Firefox), which helps you see where current CSS rules are attached.