Click on the gradient to add new color slider. Click the colored box below the slider to change color. Click the red box below indicator (and confirm) to delete. Select palette number. Click the cloud icon to upload.
Currently in use Custom Palette IDs are 0 to -1. Note!, IDs must be in uninterupted sequece from 0 and cannot excede 9. This number will only be updated on WLED reboot (or equivalent).