Taken directly from Deathau's Notation theme.
I could not find this key feature anywhere else but noticed it was implemented in Deathau's theme so I ripped it straight from there and can confirm it works great!
A must have feature if you are constantly switching focus and need to find which pane is currently active.
Just copy the below into a snippet file:
/* 7.2. Active pane border */
/* Notion doesn't have multiple panes. And with the header modifications above,
how do you tell which pane is active? I'm just putting a border around the
active pane for now (unless there's only one pane) */
.workspace-leaf.mod-active {
border: 1px solid var(--interactive-accent);
}
.workspace-leaf:not(.mod-active),
.workspace>.workspace-split>.workspace-leaf:first-of-type:last-of-type {
border: 1px solid transparent;
}