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

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

Additional Notes

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commit: 8cc3420

[nperez0111]

I'm not entirely certain about this change, the low contrast was fairly intentional to make it not look like the normal text. I am unsure that we actually need to apply the guidelines for what is essentially decorative text.

I'll let @YousefED decide here though

[YousefED]

There's a difficult trade-off here (improving accessibility vs staying more inline with other visual design decisions). My preference would be to expose this as a separate option / theme.

For now - let's hold off on merging this directly