Twitter logo in docs should not be colored

[cbrnr]

Twitter is the only colored icon on our docs website:

It should be gray like all the other icons.

[drammock]

This is inherited from the website theme. In the latest version of the theme the twitter icon color goes away (but that's actually a bug in the theme due to changing the icon from the "square" version to just the bird, but not changing the associated CSS selector). The theme also defines brand-appropriate colors for GitLab and Bitbucket. Github is grey because that's their brand.

In our MNE-Python CSS, we actually define colors for the discord icon:

mne-python/doc/_static/style.css

Line 9 in b1329c3

--mne-color-discord: #5865F2;

as well as light and dark variant colors for discourse, github, and mastadon icons, e.g.:

mne-python/doc/_static/style.css

Lines 46 to 48 in b1329c3

	--mne-color-github: rgb(240, 246, 252); /* from their logo SVG */
	--mne-color-discourse: #FFF9AE; /* from their logo SVG */
	--mne-color-mastodon: #858AFA; /* www.joinmastodon.org/en/branding */

but for some reason they aren't getting used, so that's also a bug.

Finally, one might argue we should change the twitter logo to an X logo (but that should probably happen at the theme level).

With all that said: do you still think they should be all grey, or should we try to fix the broken color display of the other icons?

[cbrnr]

Thanks for investigating @drammock! Either way is fine with me, although I have a slight preference for using color.