Default has poor visibility and system settings don’t seem to have any impact.
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What is W11?
Since you’re asking on the AppleTV sub, you can go into the AppleTV System Settings > Accessibility > Subtitles and Captioning > Style and you can change the font, font size, and font color.
Source: https://support.apple.com/guide/tvapp/activate-subtitles-and-captioning-atvb5ca42eb9/web