Hi Alexander,
There is a way to accomplish this, using subsets that represent each range you want.
For instance, if you want to create custom roof height ranges, you would first need to create subsets for each range, using filter expressions.
1) Open the model explorer
2) Click the Filter icon to create a new subset.
3) Create a filter expression for roof height. For example, if you want a range of roof heights from 1-5 units of height, then create the following filter expression: Roof Height is less than or equal to 5. In the expression editor that would look like this (below). When finished, click OK.

4) Give the new subset an easily recognizable name in the Model Explorer.

Now, open the feature themes panel, and choose that subset as a feature theme under the Feature Class drop-down menu.

5) What you have basically done is create a custom range of 1 -5 units of roof height. Specify a Quantile distribution method, then limit the number of rules to 1.

6) Now, repeat for any other custom ranges you want to create, and make sure to change the color for each new theme so they don't blend together.
For example, my next range would be Roof_Height is greater than 5 AND less than or equal to 10.

Here's the expression for that:

7) Activate visbility for any/all of the subset feature themes you've created in order to show your custom ranges.
Principal Specialist, Infrastructure