How do I access the Advanced Design Editor now? I can't seem to find it - it only has very simple them settings...

The "Advanced Designer" editor is only available in the "All Question in One Page" Form Layout. 

The current layout you are using is our new "Card Form" layout or "Single Question in One Page". If you wish to use the old layout that has the "Advanced Designer", kindly follow the steps below.

1. Go to your Form Settings.

2. Click "SHOW MORE OPTIONS".

3. Scroll down to the last page until you see the "Form Layout" field. Click the dropdown and choose the "All Question in One Page" option.

Now go to your BUILD page, now you should see your layout got changed and have the "Advanced Designer" available.

Thanks so much for the fast reply - excellent!
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You're welcome, glad to be of help.

Nice answer, I met the same problem