When first implemented, backgrounds of the minimonth calendar inside the Calendar context were removed for high contrast OS themes and situations where the OS theme is different from a set Thunderbird theme (light mode or dark mode). We need to use a variant of one of the theme colors to set a background for the calendar. It should not be transparent in these contexts.

The minimonth calendar is used throughout the application in places where a background is definitely needed for each minimonth displayed. It should not be transparent.

Inside calendar/base/themes/common/widgets/minimonth.css:

:root[systemcolors] calendar-minimonth {
  /* TODO: This should not be transparent */
  --mmMainBackground: transparent;
}

:root[lwt-tree]:not([lwt-tree-brighttext]) calendar-minimonth {
  --mmMainColor: var(--sidebar-text-color, FieldText);
  /* TODO: This should not be transparent */
  --mmMainBackground: transparent;
}

• Distinguish minimonth calendar from its container and other minimonth calendars
• Minimonth calendar should have a border in high contrast contexts
• Evaluate if a background is needed in high contrast contexts (moved to bug 1811361)

Borders and backgrounds can distinguish between different calendars and make clear the boundary of a calendar versus its container's background.

The current designs include a border, but this border was not implemented in the initial UI updates to the minimonth calendar in bug 1799934. The border needs to be implemented.

Pushed by alessandro@thunderbird.net:
https://hg.mozilla.org/comm-central/rev/84cb958ae2cc
Add border to minimonth calendar. =#thunderbird-front-end-reviewers r=Paenglab,aleca