The colour and size of the subtitles can be changed either directly in the player or in the system settings, depending on the end device or browser.
On Android and Windows (Firefox, Chrome etc.), click on the cogwheel icon in the bottom right-hand corner of the player. A small window will open in which the adjustments can be made in the subtitles area.
On iOS and macOS (Safari only), you can customise the appearance of the subtitles in the system settings under Accessibility.
Please note that some subtitles are part of the image (so-called embedded or hard subs) and cannot be individualised.