Rename scrollbar_thumb to be consistent with other style properties (#8004)

This small inconsistency was mentioned on the discord. This fixes it.

Release Notes:

- Themes: Renamed `scrollbar_thumb.background` to
`scrollbar.thumb.background` to be consistent with other style
properties.

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/theme/src/schema.rs

@@ -290,7 +290,10 @@ pub struct ThemeColorsContent {
     pub pane_focused_border: Option<String>,
 
     /// The color of the scrollbar thumb.
-    #[serde(rename = "scrollbar_thumb.background")]
+    #[serde(
+        rename = "scrollbar.thumb.background",
+        alias = "scrollbar_thumb.background"
+    )]
     pub scrollbar_thumb_background: Option<String>,
 
     /// The color of the scrollbar thumb when hovered over.