Add Style::apply()

src/theme.rs

@@ -142,6 +142,17 @@ impl Style {
 
         Ok(())
     }
+
+    /// Applies another [Style] on top of this one.
+    pub fn apply(&mut self, other: &Style) {
+        if other.fg.is_some() {
+            self.fg = other.fg;
+        }
+
+        if other.bg.is_some() {
+            self.bg = other.bg;
+        }
+    }
 }
 
 #[derive(Clone, Debug, Default)]