[A11y] Buttons become invisible if the primary color is set to white

**Describe the bug**
A clear and concise description of what the bug is.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to the site's settings `admin/settings/site` and change the `Branding > Primary color` to white RGB: (255, 255, 255)
2. Go to the admin and create a form (save it)
3. Open the public form
4. Scroll down to the `Submit` button
5. The button is almost invisible

**Expected behavior**
The button should still be visible and have a contrast, even if the background is the same color.

**Screenshots**

and highlighted: