Custom Theme: Adding a Border Next to Layer Name

I’m really happy with Aseprite’s default dark theme. However, in some custom themes, there’s a border adjacent to each layer’s name, and I like how it clearly indicates when layers are nested.

I’d like to add this border to Aseprite’s dark theme myself. Looking in the theme.xml file, I can’t find the matching property. Any help on locating it?

Layer Border

Screenshot taken from KashouC’s darktheme.