Add ui.gutter.selected option for themes (helix-editor#3303)

* add `ui.gutter.selected`

* add `ui.gutter`, `ui.gutter.selected` to docs

Author: ChrHorn

book/src/themes.md

@@ -219,6 +219,8 @@ These scopes are used for theming the editor interface.
 | `ui.cursor.select`        |                                                |
 | `ui.cursor.match`         | Matching bracket etc.                          |
 | `ui.cursor.primary`       | Cursor with primary selection                  |
+| `ui.gutter`               | Gutter                                         |
+| `ui.gutter.selected`      | Gutter for the line the cursor is on           |
 | `ui.linenr`               | Line numbers                                   |
 | `ui.linenr.selected`      | Line number for the line the cursor is on      |
 | `ui.statusline`           | Statusline                                     |

helix-term/src/ui/editor.rs

@@ -745,6 +745,7 @@ impl EditorView {
         let mut offset = 0;
 
         let gutter_style = theme.get("ui.gutter");
+        let gutter_selected_style = theme.get("ui.gutter.selected");
 
         // avoid lots of small allocations by reusing a text buffer for each line
         let mut text = String::with_capacity(8);
@@ -757,6 +758,12 @@ impl EditorView {
                 let x = viewport.x + offset;
                 let y = viewport.y + i as u16;
 
+                let gutter_style = if selected {
+                    gutter_selected_style
+                } else {
+                    gutter_style
+                };
+
                 if let Some(style) = gutter(line, selected, &mut text) {
                     surface.set_stringn(x, y, &text, *width, gutter_style.patch(style));
                 } else {