I tried to follow these steps to create a custom color palette, but it did't pick up any change in colors. The color theme is available in the app options. I'm not sure if i have the correct contents in the json file or just an Qlik version that doesn't support it.
I'm using Qlik Sense 13.9.1 (about a year behind now)
The associated files are below. I have only changed the first 7 colors, everything else should be the same as the files in the examples.