Update styling of selected line

What does this MR do and why?

• Adds top and bottom border to selected line rows and commented line rows
• Changes commented line row color from yellow to blue
• Removes border between changed line numbers
• Darkens line number font color

TODO:

• Check if purple is okay or we should use yellow/orange (or blue)
• Update all themes
• Update file view styles
• Remove double border when hovering on comment
• Fix multi-line comments
• Check custom diff colors

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Go to merge request
• Select a line
• Add a comment (and hover over comment; lines should be highlighted)
• Add a multi line comment

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #384074 (closed)