Problem/Motivation
Similar to the issue reported in https://www.drupal.org/project/drupal/issues/2930508, the contrast ratio between the text and background color on inactive tabs is 4.21. Should be 4.5 or higher.
Steps to reproduce
1. Log into Drupal with the Seven admin theme selected
2. Go to any page using vertical tabs
3. Run an accessibility checker. Observe that the color contrast is flagged.
Proposed resolution
Adjust the background color to be a little lighter, increasing contrast, and add an underline to the text on the active tab so there is an indicator other than color.