frontend/themes: Re-add transparent border

It appears that in conjunction with other changes that happened around
the time the removal of the transparent border got merged, it became
necessary again due to those other changes. Without it, only the
selected and focussed changes have a border, making the content jump
around when selected and focussed.

Reverts part of 5fa4ea44d0b364bf9be18816c2d9b35b96bab939.

frontend/data/themes/Yami.obt

@@ -530,6 +530,7 @@ QListWidget::item,
 QMenu::item {
     border-radius: var(--border_radius);
     color: var(--text);
+    border: 1px solid transparent;
 }
 
 SourceTreeItem {