Add colours for new Discord themes

[LeoCx1000]

Summary

Adds the following:

• Colour.ash_theme
• Colour.ash_embed
• Colour.onyx_theme
• Colour.onyx_embed
• Colour.light_theme

Updates the following:

• Colour.dark_theme
• Colour.dark_embed
• Colour.light_embed

I believe these are all correct, however if you can, please double-check these values.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)