Minor design adjustments

[kjellr]

This PR makes a handful of small design updates:

• Adjusts the light gray color to pass WCAG AA
• Adds formatting to the Entry Meta below the post title
• Fixes position of the posts navigation on mobile
• Lightens border colors around the entry-footer and post navigation
• Properly styles the post footer (this was using opacity before for some reason).
• Removes visited link color styles from dark mode (since we don't use them in Gutenberg, or in light mode)

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Adjusts the light gray color to pass WCAG AA, and adds formatting to the Entry Meta below the post title:

Before

After

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Fixes position of the posts navigation on mobile, lightens border colors:

Before

After

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Avoids using opacity styles on the footer:

Before

After