How to set theme?

[B.O.S.S]

Hello, I wanted to set custom theme by Ventoy Plugin but ventoy is still using own theme. Here is my configuration: On first partition (where are iso's) I have folder named ventoy, inside of it there is ventoy.json with content: 

Code:

{
    "theme": {
        "file": "ventoy/Vimix/theme.txt"
    }
}

Next to that file there is folder "Vimix" it's grub theme folder. That theme worked when i set it by changing grub.cfg in 2nd partition.
Here is also output of tree command from first partiton
What is wrong with my configuration?