Interactive Video Button Colours

I would like to know how to change the colour of the Text Icon used in the Interactive Video element. I am currently using H5P with wordpress plugin on my website. I am also using a theme which allows me to add custom CSS.

Due to my limited knowledge of CSS I am unable to find the CSS to change this.

The default icon is shown in this image:

Ideally, I would like to change it to a transparent image that I can place over the top of a word or another icon or image that is already in my video.

Any help is appreciated.


Change colour of text and image button in interactive videos