Text and background/ highlight color not able to change

I am unable to change the colour of the text and highlight/background in the twenty-twenty theme.

When a text has a link, it automatically gets a background/highlight colour, which is default a yellow highlight. I have changed it to white, but now I want an image as a background for the whole website, and I want the highlight to become transparent. When I select the text, click “highlight”, click the box where you can choose the colour for background, and click the lines to be able to write in the exact colour either in hex, RGB or HSL it does not allow me to do this, so I can not choose custom colours. I think it is the same problem this person had: https://wordpress.com/forums/topic/unable-to-change-text-color-anymore/, so it seems like a bug, but would like to know how one or if one can fix it?

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