fix readability of medium-editor toolbar

[sa-rang]

Fixes #1003

$medium-editor-background-color: var(--va-background-primary);

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)