Add style class for CPU state

Fixes: https://github.com/Alexays/Waybar/issues/2911

include/modules/cpu.hpp

@@ -22,6 +22,7 @@ class Cpu : public ALabel {
 
  private:
   std::vector<std::tuple<size_t, size_t>> prev_times_;
+  std::string prev_state_;
 
   util::SleeperThread thread_;
 };

man/waybar-cpu.5.scd

@@ -121,3 +121,5 @@ CPU usage per core rendered as icons:
 # STYLE
 
 - *#cpu*
+- *#cpu.<state>*
+	- *<state>* can be defined in the *config*. For more information see *states*.

src/modules/cpu.cpp

@@ -36,6 +36,12 @@ auto waybar::modules::Cpu::update() -> void {
     format = config_["format-" + state].asString();
   }
 
+  if (!prev_state_.empty()) {
+    label_.get_style_context()->remove_class(prev_state_);
+  }
+  label_.get_style_context()->add_class(state);
+  prev_state_ = state;
+
   if (format.empty()) {
     event_box_.hide();
   } else {