Tile board: How to change tile background based on value

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Hi all, I have a tile:

{
position: [7, 3],
type: TYPES.SENSOR,
title: ‘Winnie Track’,
id: ‘device_tracker.ggqgcvpn’,
unit: ‘%’, // override default entity unit
value: ‘@attributes.battery_level’,
},

And would like to change the tile background colour when the value is < 25%. Ive messed around with custom_styles, but cant figure out the function to do this. Any ideas? Thanks.