Brand guide alignment

Some points where the website needs better alignment with [brand guide](https://raw.githubusercontent.com/jupyter/design/main/brandguide/brand_guide.pdf) (or branding guidelines maybe should be adjusted?):

1. The footer of the website uses the black logo on orange, but pages 8-9 of the brand guide says to use the white logo
2. p. 5 says "Jupyter Orange is the primary brand color. It should never be used in tints or shades." However we have a number of different orange tints:
- buttons are a darker orange
- the footer has several shades of orange (Perhaps we should switch the footer to use shades of dark grey?)
- https://github.com/jupyter/jupyter.github.io/blob/fb6c137ab177fae9363ef9e633bad9c7f4669df6/_sass/settings/_colors.scss#L3
- https://github.com/jupyter/jupyter.github.io/blob/fb6c137ab177fae9363ef9e633bad9c7f4669df6/_sass/settings/_colors.scss#L19-L21

Read the linked brand guide, especially pages 5 and 8-9, then inspect the footer and the cited entries in _sass/settings/_colors.scss. Compare the current logo and orange shades with the guide and identify the website elements that need alignment. Done means the affected branding choices consistently follow the agreed guidance.

Written by the indexing model from the issue text.