There are many different options for displayed captions in YouTube like font color, font size, font type, background color and so on.
Here is how to change these from the default ones displayed :
Click on the caption icon (rectangle with lines) for any YouTube video that has them enabled.Â Next, go to Options.
This will popup a window with various caption attributes.
Choose as per preferences and hit “Done” or “Reset” to revert back to default settings.