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Hex Number Obfuscation
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You can not pass a hex-number-obfuscated string to insertRule()
because it converts back to a normal string so when you see it
in DevTools, you won't see hex numbers.

You see hex numbers in DevTools only when the css is loaded as
text as an inline style or external style sheet.

So, you should give up combining the JavaScript obfuscation
using insertRule and hex number obfuscation.

If you want to use hex number obfuscation to obfuscate CSS,
then offer it as non-JavaScript solution.

The obfuscator will be a webpage application that runs
in a local environment only. It takes CSS code as an input,
and outputs hex-obfuscated code.

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Constructed Stylesheet
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The good news is that a stylesheet created using new CSSStyleSheet()
can accept and retain hex number obfuscation.
(Chrome only, it won't work in FF)

The bad news is you can insert it into DOM using `adoptedStyleSheets` only.
`adoptedStyleSheets` will put it after all <style> so you can not insert it
at the location you want.
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<script>
"use strict";
const sharedSheet = new CSSStyleSheet();
sharedSheet.replace('.mydiv { \\63olor: \\67\\72\\65\\65\\6e; }');

// Apply the same stylesheet to the document
document.adoptedStyleSheets = [sharedSheet];
</script>

<style>
.mydiv { color: red; }
</style>

<div class="mydiv">This is a test sentence.</div>
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