Twenty Twenty-One: Menu button has the wrong colors

[poena]

An unintended side effect of the updated button styles in #51927 was that the menu button changed color.

In trunk, the current menu button has a dark grey background color
var(--global--color-primary); and a transparent text.

As per design, the background should be transparent and the text should be dark grey.

[poena]

Menu button with incorrect colors

[poena]

Override specificity from default button styles by using the button ID.

[SergeyBiryukov]

In 50026:

Twenty Twenty-One: Correct colors for the Menu button.

As per design, the background should be transparent and the text should be dark grey, not vice versa.

Follow-up to [49987].

Props poena.
Fixes #52374. See #51927.

[jeroenrotty]

Mobile menu button with correct styling

[jeroenrotty]

Tested the patch with @rolfsiebers and the patch worked as intended, see screenshot.