I do not think that there is a widespread method for constructing such forms with pure css.
What could you try using inline svg:
background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='10' height='10'><linearGradient id='gradient'><stop offset='10%' stop-color='%23F00'/><stop offset='90%' stop-color='%23fcc'/> </linearGradient><rect fill='url(%23gradient)' x='0' y='0' width='100%' height='100%'/></svg>");
This is just an example of svg, you have to model yourself. It also takes base 64:
background-image: url(data:image/svg+xml;charset=utf-8;base64,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) no-repeat;
Edit: I created a fiddle: https://jsfiddle.net/pm3czdhj/7/ You can also try looking at the css clip-path property.
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