River/tags: Add static CSS classes for all available tags.

man/waybar-river-tags.5.scd

@@ -50,6 +50,7 @@ Addressed by *river/tags*
 - *#tags button.occupied*
 - *#tags button.focused*
 - *#tags button.urgent*
+- *#tags button.tag-N*
 
 Note that occupied/focused/urgent status may overlap. That is, a tag may be
 both occupied and focused at the same time.

src/modules/river/tags.cpp

@@ -149,6 +149,7 @@ Tags::Tags(const std::string& id, const waybar::Bar& bar, const Json::Value& con
         button.signal_button_press_event().connect(
             sigc::bind(sigc::mem_fun(*this, &Tags::handle_button_press), (1 << tag)));
     }
+    button.get_style_context()->add_class("tag-" + std::to_string(tag + 1));
     button.show();
   }