fix(colour): Fix dark_theme color

[bastigamedc]

Summary

In this PR I set the color of dark_theme to the new dark_theme color of Discord

Information

• 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, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
• I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes. (I don't know where to update this)

[Lulalaby]

Do we even need that anymore. There's too many "dark themes" now. Even without nitro. /gen Q for @Pycord-Development/contributors

[bastigamedc]

Or should I add an individual function for each theme?

[plun1331]

This color is completely irrelevant now anyways with components v2

[JustaSqu1d]

if we were to go ahead with this PR, then we should have the previous dark color in a field called old_dark_theme or something

[NeloBlivion]

i think "dark_theme" should be left as is and deprecated in favour of the official names "dark", "onyx" and "ash"

[Lulalaby]

I think we should get rid of all these "theme" things. I see no use in it anyways

[Paillat-dev]

The use afaik is for making it blend with the bg

[plun1331]

The use afaik is for making it blend with the bg

There are too many backgrounds for it to be reasonable anymore (also no gradients)
The best solution if you really want this functionality would be to use cv2