The workspaces navigation block button has several color contrast related issues in light mode

Problem/Motivation

There are two problems. First in light mode the button color #88cc5f against #ffffff has only a contrast of 1,93:1 the required minimum is at least 3:1 to comply with SC1.4.11. in dark mode the contrast of the button #31cea4 is fine against the background #2a2a2a leading to a contrast of 7,15:1. The second problem is the color contrast of the button label against the button background due to the fact that the button label in light mode is using the accent color for coloring the label while in dark mode the label color is always dark no matter which accent color is chosen. blue, light blue, purple, teal, green, pink, red, orange, yellow have a color contrast < 4.5:1 against the green button in light mode. only dark purple and neutral have a contrast >4.5:1 that complies with SC1.4.3.

Steps to reproduce

Go to admin/appearance/settings/default_admin and switch to light mode and then between accent colors with the workspaces module installed and watch the color contrast

Proposed resolution

1. Increase the contrast for the green of the workspaces nav block button against the background of the navigation sidebar in light mode to at least 3:1
2. Drop the usage of accent colors on the workspaces nav block button labels in light mode and just use a dark color same as in dark mode. (discussed the approach dropping the accent color with @kentr)

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet