Change CSS colour to rebeccapurple

[DecimalTurn]

Description

This PR changes the CSS color to be in line with the new CSS logo that was recently adopted.
The discussion related to the change took place in a GitHub issue (see link below) and the new color got almost 500 🎉, so the consensus is there.

Also, the change remains minor since this is a change from a shade of purple to another. However, for people that know about the meaning of rebeccapurple, it's a nice touch.

Current: #563d7c
Suggested: #663399

Fixes #7132

Checklist:

• I am changing the color associated with a language
  • I have obtained agreement from the wider language community on this color change.
    • new CSS logo? CSS-Next/css-next#105 (comment)

[Alhadis]

@lildude Community consensus checks out. Also, the colour change itself is scarcely noticeable (at least to my deuteranopic eyes…)

[lorypelli]

I hope this PR gets merged, that color has an important meaning that is really touching...🥹

[lorypelli]

@lildude Community consensus checks out. Also, the colour change itself is scarcely noticeable (at least to my deuteranopic eyes…)

I think that changing it for the meaning of the color is more important than the fact that is not noticeable