Twenty Twenty-One: Menu button has the wrong colors

[poena]

An unintended side effect of the updated button styles in https://core.trac.wordpress.org/ticket/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.