Adapt messages table to dark mode

[AntonKhorev]

• I remove the custom unread message background and replace it with table-success. The custom background was applied directly to table cells which we presumably don't want. This changes the green color used in the table.
• I extend dark mode overrides to cover table-success.
• I remove the inbox-row class. It was also used in the outbox.

Light mode before:

Light mode after:

Dark mode after:

[tordans]

Follow up: I think from an accessibility point of view we should consider encoding the information that we (only) present via background colors with text/html parts. Some ideas…

• add hidden columns for screen reader only that have text describing the "read/unread" state
• use some aria property on the row to provide this read/unread status https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes

[tomhughes]

I'm not sure about this one - the green is a bit pale for my tastes but I could probably live with that but what I really find a bit odd is the way the highlighted rows get a bolder lower border because table-success also changes the border colour.

[AntonKhorev]

Lowered $table-border-factor:

[tomhughes]

That looks much better, thanks.