https://chromewebstore.google.com/detail/rakuten-tv-substyler-cust/gnpcigljiekghnhokkfmpjnodcgoehfe
Rakuten TV SubStyler: customize subtitles - Chrome Web Store
Extension to customize captions and subtitles on Rakuten TV. Change text size, font, color, and add background.
rakuten tvchrome webcustomizesubtitlesstore