I like the styles you have chosen, very original. Just a few issues underneath the hood.
When I increase the text size the calculator gets wider and the buttons spaced further apart, to the point that it creates a horizontal scroll bar.
When I make the browser window shorter and cause the display to go above the top of the viewport, I am not able to scroll high enough to get the display back.
At the default font size, the dark text on red buttons is not quite accessible (not enough contrast). If you make the text pure black (#000) then it will be accessible. You can check contrast accessibility at https://webaim.org/resources/contrastchecker/
I think the keyboard focus indicator needs to be more prominent. I can barely tell where my focus is.