Potentially use system fonts/colors

[Malvoz]

Investigate if/where to use:

System colors

• LinkText is currently used for interactive features (stroke)
• The SVG for markers should perhaps default to LinkText (fill, stroke)
• Visited map links (map-a-visited) should probably use VisitedText (stroke)
• Appropriate system colors for the mapml-link-preview (same as browsers' URL preview)?
• Appropriate system colors for the tooltips (same as browsers' tooltips from e.g. title attribute)?

System fonts

• Maybe use the menu font value for the context menu.

There's also the related system-ui font.

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This issue relates to #551 (dark mode); if we use system colors instead of arbitrary fixed colors we in theory support dark mode (and high contrast mode) out of the box.

Other resources

• https://blog.jim-nielsen.com/2021/css-system-colors/
• https://www.stefanjudis.com/today-i-learned/css-defines-color-values-that-follow-system-preferences/
• http://www.iangraham.org/books/xhtml1/appd/update-23feb2000.html