If you are running into an error saying n.replaceAll is not a function (or o.replaceAll) when trying to use Theme Designer, this is very likely happening due to the relatively old age of your browser (e.g. Chrome 80 and below).
The Theme Designer feature relies on features that only function in more modern browser versions.
To resolve this, we strongly recommend looking into updating your browser to a newer version, if only for your own online security!