CSS Styling for the sidebar

For this PR, I've added styling to our sidebar. Here is a list of what has been added with some visual examples later on:

• sidebar colour now matches figma mocks
• sidebar height grows with its parent
• sidebar has a styled scrollbar
• icons are set for the nodes
• selected node has appropriate highlighting
• context menu has some styling as well (this was done by eye since we don't have any figma mocks for this yet)