You are correct, there is no modify command. The only way I know to do this is to be in a level one paragraph and use the Define New MultiLevel List dialog. It will start with the current list and let you make modifications. (The single-level list dropdown does have a Modify command, but does not work on a multilevel list and even fyor a single-level list may be that dialog to use because it has so many more formatting controls.)
If you use automatic paragraph numbering or bullets read Shauna Kelly's directions on numbering and bullets. Start with How to create numbered headings or outline numbering in Word 2007 and Word 2010. For large documents you must follow these directions or you will lose your hair!
(Mac version: https://www.brandwares.com/bestpractices/2016/06/outline-numbering-in-word-for-os-x/). For styles attached this way, you also control the left indents through the Define New MultiLevel List dialog not the Ruler or the Modify Style dialog.
This may seem a bit convoluted at first, but it really is not. Just follow the steps. Shauna Kelly's instructions use the built-in heading styles, but you can use any existing paragraph styles including your custom styles. There are, however advantages to using the built-in heading styles when you create a Table of Contents. Here are some more advantages: Why Use Word's Built-In Heading Styles? by Shauna Kelly Note, you can modify these built-in styles to look exactly the way you want.
You want to do this even if what you want is a single-level list if you want the most control over your list.
The basic idea is that the numbering is set using the Define MultiLevel List dialog with each numbering level being attached to an existing paragraph style. Once you have this set up, you should not use the buttons for numbering in the Ribbon but rather apply the appropriate style for that level.
You can save a document with this as a template for future documents if you want so you will not need to do this every time.
Styles in a multilevel list, when properly set up, can be a part of a [Quick] Style Set or applied by a macro using the Organizer method.