Changes diff table colors and padding

[trevorshannon]

This PR:

• Adjusts diff highlight colors used in the HTML table diff to match those used on github.com diff views.
• Includes support for dark mode by specifying alternate colors via the prefers-color-scheme: dark media query.
• Adds padding to the diff table for improved readability
• Selectively hides table borders for a cleaner view

The following screenshots were made on MacOS running Chrome.

Before (light mode)

After (light mode)

Before (dark mode)

After (dark mode)

Fixes #729

[thp]

Nice and clean. Thanks for the screenshots in the description as well!