Autumn theme: improve markup highlighting

[getreu]

This commit mainly improves markup highlighting.

• Italic and bold are displayed as italic and bold without changing the font colour.
• URLs get their own colour.
• The lightness of my_green is slightly adjusted for better readability.

A new colour my_yellow2 is introduced to distinguish warnings and methods visually more easily.

[the-mikedavis]

looks good 👍

You may also want to check out theming headers in markup (example), so you can have different colors for each level of header. Most themes don't have it but it's configurable.