Here is a conversation about it
TLDR;
I created a role
by adding the following to my native libraries:
<style>
.anvil-role-bottom_thin_border {
border-bottom: thin solid;
}
</style>
Then I created a role called bottom_thin_border
in the theme/role section of the IDE.
If you don’t restrict the role, you can then apply it to any component using the role drop-down in the appearance section of the component properties.
or via code
self.component.role = "bottom_thin_border"