Adjust color of chapter markers on OSD, fixes #16

[robert-corlett]

Fixes #16 by adjusting the color of the chapter markers to blue to improve contrast and ensure visibility across both windowed and fullscreen OSC. Only applies to dark mode.

• [ ] This change has been discussed with the author.
• [x ] It implements / fixes issue #16 .

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Description:

• Adjusts the color of chapter markers to blue when in 'dark' mode, which improves visibility across both windowed and fullscreen contexts and fixes the zero-visibility issue references in issue #16
• Darkens the color of chapter markers slightly when in 'light' mode to ensure sufficient contrast.

Please forgive any n00b errors I may have made re: commit/pull request etiquette -- this is my first toe-dip.

[CarterLi]

I don't think the contrast is sufficient enough.

[CarterLi]

cherry-picked