fix: Default themes contrast

[matthewlipski]

Summary

Currently, the the value set for the --bn-colors-side-menu CSS variable in the default light and dark themes, has a contrast that is too low relative to the --bn-colors-editor-background variable.

This is relevant because it's used for the side menu and block placeholder text colors, and doesn't comply with WCAG accessibility standards.

This PR changes the value of --bn-colors-side-menu to comply with the WCAG 2 AAA standard.

Closes #1983

Rationale

Useful improvement for vision impaired users.

Changes

Updated --bn-colors-side-menu CSS variable value for light and dark default themes.

Impact

N/A

Testing

N/A

Screenshots/Video

N/A

Checklist

• [X] Code follows the project's coding standards.
• [X] Unit tests covering the new feature have been added.
• [X] All existing tests pass.
• [X] The documentation has been updated to reflect the new feature

Additional Notes