The following tutorial helps you add custom icons for individual layers
From ARki 7.0 customise layer icons using the new custom icon feature.
1: Touch and hold the layer button to open the custom icon menu
2: Select the image button and choose between preset or custom icons
3: For custom icons import JPG or PNG file below 1mb and 512 px wide
Model credit: Nextgen railway footbridges by Network Rail Buildings and Architecture