Thanks for changing the indexing to include topic names that begin with numbers. Numbered items, then, are all under the Index Section Heading: "1".
Changing the text style for all of the Index Section Heading items changes them all - except the "1"
So all section headings A-Z can be changed to have a different font size. But the "1" (that is also marked as "Index Section Heading") stays the same size. Example: "A-Z" are 5 points, "1" is 12 points.
See screenshots.