Twenty Nineteen: The navigation block submenu button is unreadable

[poena]

The navigation block has an option called "Open on click" that is available when there is a submenu.
When the option is enabled, the parent menu item is a <button> element.
On the front of the website, this button inherits the background styles from the themes button CSS:

button:hover, button:hover, input[type="button"]:hover, input[type="reset"]:hover, input[type="submit"]:hover {
	background: #111;
	cursor: pointer;
}

So when the themes default colors are used and someone hovers over the button, the background and text has the same colors, and the text is not readable.

Testing instructions:

Activate Twenty Nineteen
Make sure that your WordPress installation has some content that you can place in the navigation block, because you will need to create a submenu.

Create a new post or page.
Add a navigation block. In the block, select the inserter and add a link: this will be your parent menu item. Click on the link and select the option "Add submenu". Add a link.

Save.
Go to the front of the website.
Locate the navigation and hover over the link item that has the submenu.
Confirm that both the text and the background are dark grey, close to black.

[sabernhardt]

Duplicate of #59924.