Updated on Fri Dec 25 2020
<style>
:root {
--my-color: #f00;
}
</style>
<canvas id="my-canvas"></canvas>
<script>
const canvasElement = document.getElementById('my-canvas')
const ctx = canvasElement.getContext('2d')
ctx.fillStyle = window.getComputedStyle(document.documentElement).getPropertyValue('--my-color')
ctx.fillRect(10, 10, 150, 100)
</script>
Thanks for the tip, @edokoa!
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