chore: s/customStyle/custom properties?

fix: do you mind testing the match using regex since startsWith is not supported in IE?

// regex that matches custom property or no hyphen
var CUSTOM_PROPERTY_OR_NO_HYPHEN_REGEX = /^--[a-zA-Z]+$|^[^-]+$/;

function camelCase(string) {
  // ...

  if (CUSTOM_PROPERTY_OR_NO_HYPHEN_REGEX.test(string)) {
    return string;
  }

  // ...

seems to not work with custom styles having lisp-case/dash-case in between ...

for example:

CUSTOM_PROPERTY_OR_NO_HYPHEN_REGEX.test('customStyle') // returns : true
CUSTOM_PROPERTY_OR_NO_HYPHEN_REGEX.test('--customStyle') // returns : true
CUSTOM_PROPERTY_OR_NO_HYPHEN_REGEX.test('--custom-style') // returns : false

I think it should also be possible to use custom style having dashes in between (e.g. '--custom-style'). Therefore lets consider an easier approach:

// customStyle or no hyphen found
  if (string.indexOf('--') === 0 || string.indexOf('-') === -1) {
    return string;
  }

It could be possible to use a regex for this check, or to directly replace the string by camelCase if needed, but since javascript does not fully support lookbehind-assertions, the regex gets a little tricky ...

Good point about the lisp-case. What about the modified regex?

-var CUSTOM_PROPERTY_OR_NO_HYPHEN_REGEX = /^--[a-zA-Z]+$|^[^-]+$/;
+var CUSTOM_PROPERTY_OR_NO_HYPHEN_REGEX = /^--[a-zA-Z-]+$|^[^-]+$/;

The indexOf also works, but do you think it's worth making the declaration value check strict?