src/vs/workbench/services/themes/common/colorThemeData.ts/hexUpper
function hexUpper(charCode: CharCode): number {
	if (charCode >= CharCode.Digit0 && charCode <= CharCode.Digit9 || charCode >= CharCode.A && charCode <= CharCode.F) {
		return charCode;
	} else if (charCode >= CharCode.a && charCode <= CharCode.f) {
		return charCode - CharCode.a + CharCode.A;
	}
	return 0;
}