This topic was helpful.
This is what I did:
For removing or changing properties of the separator: In Tree View, look under Gallery for a Separator.
Option 1: Clicked on it to modify its properties.
Option 2: Remove it and add alternate color to each rows in the Gallery. For alternate colors in the Gallery, find a sequential number in your data or while fetching the data add a column with sequential number - then use Mod function with 2 to identify Odd/Even number and use a condition to apply changing colors for each row with a if condition under TemplateFill property.
[My example: If(Mod(ThisItem.SeqID,2)=0,RGBA(0, 0, 0, 0.2),RGBA(0, 0, 0, 0)) ]