Add player colors to serialized themes (#6887)

This PR adds player colors to serialized themes.

Release Notes:

- N/A

crates/theme/src/schema.rs

@@ -59,6 +59,9 @@ pub struct ThemeStyleContent {
     #[serde(flatten, default)]
     pub status: StatusColorsContent,
 
+    #[serde(default)]
+    pub players: Vec<PlayerColorContent>,
+
     /// The styles for syntax nodes.
     #[serde(default)]
     pub syntax: IndexMap<String, HighlightStyleContent>,
@@ -1153,6 +1156,13 @@ impl StatusColorsContent {
     }
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
+pub struct PlayerColorContent {
+    pub cursor: Option<String>,
+    pub background: Option<String>,
+    pub selection: Option<String>,
+}
+
 #[derive(Debug, Clone, Copy, Serialize, Deserialize, JsonSchema)]
 #[serde(rename_all = "snake_case")]
 pub enum FontStyleContent {