Increase contrast in dark-mode (#38525)

[kusma]

Description

This increases the default brightness for the body text-color in dark mode, which inceases readability a bit.

It was also suggested to darken the background a bit, but we unfortunately don't have a darker gray in our color palette. So for now, let's increase the text-color by two notches, and leave the background color as-is instead.

Motivation & Context

See issue #38525

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

(not really sure any of the above applies here)

Live previews

• https://deploy-preview-38815--twbs-bootstrap.netlify.app/

Related issues

#38525

[kusma]

This matches the work-around I implemented for mesa3d.org.

[kusma]

Worth to note, this increases the contrast ratio from 8.22 to 13.1.

[pranjal32]

The original contrast really helps to differentiate between body text and headings as a dual tone effect, however with everything as white text, there seems to be a bit of blending.

[kusma]

The original contrast really helps to differentiate between body text and headings as a dual tone effect, however with everything as white text, there seems to be a bit of blending.

It's not white, it's just a bit closer to white.

FWIW, the contrast between the body-text and the headings are now more similar to what we do for light mode, which seems like a good thing to me:

[mdo]

This feels like a great first start to me, thanks!