This button is picking up its background colour from the accent colour.
If I create a new button, I get what you see above.
So I go to Site Settings and the Buttons tab and set the background colour
I would expect this to set the background colour of all buttons to the light blue colour, but it doesn’t. They stay gold.
Of course, I can go to the individual button settings and change the colour, but strangely the Clear button does not work.