Indigo layout index has keywords and subkeywords at same level

I'm using the Indigo HTML 5 layout in RH2017. The index in my help project contains keywords and subkeywords. However, the subkeywords are listed at the same level as the keyword. This causes the index to look like it's in non-alphabetical order, because it's not clear to the user which are keywords or subkeywords.

Here is a screenshot with the Indigo layout:

And here is the same index in a CHM output. The subkeywords are at a different level.

Is there a way to reproduce the CHM-style nesting of the index with the Indigo layout?