This affects Linux and Windows.

@michailbery first found this bug on Linux during review of patch D168853 for bug 1804488. He wrote: "In dark mode on Linux when hovering over the buttons you get a kinda double border and no hover highlight color."

In Windows with a system dark theme and Thunderbird inheriting the theme, the button hover background doesn't change and all that is seen is a very slight color change in the border. This is not enough contrast change.

Expected behavior:

• On hover, the button should have a background color change
• On hover, the button should have a single border
• On hover, the border, larger container background, and button background should be enough color contrast to distinguish between the button and its containing element

Work so far:
@aleca suggested a change in patch D168853, but applying this change affected Windows system light theme with Thunderbird set to a dark theme.

Patch D170954 began to address a solution by aligning the styling of the minimonth header to match how widgets.css buttons work (in terms of hover colors).

Planned work:
Patch D170954 will be moved to this bug from bug 1804488.

In dark mode, adjust hover color

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/5fe69dce08f8
Adjust hover on minimonth header buttons. r=aleca