Bug 30952: Harmonize tabs style

Bug 30952: Harmonize tabs style

- Reduce border width from 3px to 2px
- Active tab is white on "primary" green (WCAG AA ✓)
- Other tabs are black (#111) on "secondary" green (WCAG AA ✓)
- Font weight is normal

The patch also removes some useless/redundant CSS properties related to
tabs

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>