power: Use correct colour for battery level bars

Use 3 symbolic colours to paint the levels in the battery bars, with a
red "error" colour used for the lowest level of battery, an orange
"warning" colour for the pre-error level, and a green "success" colour
used for levels above that.

There's no yellow intermediate colour as this is usually too anxiety
inducing and there's no real need to press users into a "warning"
behaviour when the level will still be comfortable for a long enough
time.

Closes: #725

panels/power/battery-levels.css

@@ -0,0 +1,14 @@
+levelbar block.warning-battery-offset {
+  background-color: @error_color;
+  border-color: @error_color;
+}
+
+levelbar block.low-battery-offset {
+  background-color: @warning_color;
+  border-color: @warning_color;
+}
+
+levelbar block.high-battery-offset {
+  background-color: @success_color;
+  border-color: @success_color;
+}

panels/power/cc-power-panel.c

@@ -446,6 +446,19 @@ get_details_string (gdouble percentage, UpDeviceState state, guint64 time)
   return details;
 }
 
+static void
+load_custom_css (CcPowerPanel *self)
+{
+  g_autoptr(GtkCssProvider) provider = NULL;
+
+  /* use custom CSS */
+  provider = gtk_css_provider_new ();
+  gtk_css_provider_load_from_resource (provider, "/org/gnome/control-center/power/battery-levels.css");
+  gtk_style_context_add_provider_for_screen (gdk_screen_get_default (),
+                                             GTK_STYLE_PROVIDER (provider),
+                                             GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+}
+
 static void
 set_primary (CcPowerPanel *panel, UpDevice *device)
 {
@@ -492,9 +505,9 @@ set_primary (CcPowerPanel *panel, UpDevice *device)
   levelbar = gtk_level_bar_new ();
   gtk_widget_show (levelbar);
   gtk_level_bar_set_value (GTK_LEVEL_BAR (levelbar), percentage / 100.0);
-  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), GTK_LEVEL_BAR_OFFSET_LOW, 0.03);
+  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), "warning-battery-offset", 0.03);
-  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), GTK_LEVEL_BAR_OFFSET_HIGH, 0.1);
+  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), "low-battery-offset", 0.1);
-  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), GTK_LEVEL_BAR_OFFSET_FULL, 0.8);
+  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), "high-battery-offset", 1.0);
   gtk_widget_set_hexpand (levelbar, TRUE);
   gtk_widget_set_halign (levelbar, GTK_ALIGN_FILL);
   gtk_widget_set_valign (levelbar, GTK_ALIGN_CENTER);
@@ -603,9 +616,9 @@ add_battery (CcPowerPanel *panel, UpDevice *device)
   levelbar = gtk_level_bar_new ();
   gtk_widget_show (levelbar);
   gtk_level_bar_set_value (GTK_LEVEL_BAR (levelbar), percentage / 100.0);
-  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), GTK_LEVEL_BAR_OFFSET_LOW, 0.05);
+  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), "warning-battery-offset", 0.05);
-  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), GTK_LEVEL_BAR_OFFSET_HIGH, 0.1);
+  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), "low-battery-offset", 0.1);
-  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), GTK_LEVEL_BAR_OFFSET_FULL, 0.8);
+  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (levelbar), "high-battery-offset", 1.0);
   gtk_widget_set_hexpand (levelbar, TRUE);
   gtk_widget_set_halign (levelbar, GTK_ALIGN_FILL);
   gtk_widget_set_valign (levelbar, GTK_ALIGN_CENTER);
@@ -793,9 +806,9 @@ add_device (CcPowerPanel *panel, UpDevice *device)
   gtk_widget_set_halign (widget, GTK_ALIGN_FILL);
   gtk_widget_set_valign (widget, GTK_ALIGN_CENTER);
   gtk_level_bar_set_value (GTK_LEVEL_BAR (widget), percentage / 100.0f);
-  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (widget), GTK_LEVEL_BAR_OFFSET_LOW, 0.03);
+  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (widget), "warning-battery-offset", 0.03);
-  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (widget), GTK_LEVEL_BAR_OFFSET_HIGH, 0.1);
+  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (widget), "low-battery-offset", 0.1);
-  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (widget), GTK_LEVEL_BAR_OFFSET_FULL, 0.8);
+  gtk_level_bar_add_offset_value (GTK_LEVEL_BAR (widget), "high-battery-offset", 1.0);
   gtk_box_pack_start (GTK_BOX (box2), widget, TRUE, TRUE, 0);
   gtk_size_group_add_widget (panel->level_sizegroup, widget);
   gtk_box_pack_start (GTK_BOX (hbox), box2, TRUE, TRUE, 0);
@@ -2533,6 +2546,7 @@ cc_power_panel_init (CcPowerPanel *self)
   g_resources_register (cc_power_get_resource ());
 
   gtk_widget_init_template (GTK_WIDGET (self));
+  load_custom_css (self);
 
   cc_object_storage_create_dbus_proxy (G_BUS_TYPE_SESSION,
                                        G_DBUS_PROXY_FLAGS_NONE,

panels/power/power.gresource.xml

@@ -2,5 +2,6 @@
 <gresources>
   <gresource prefix="/org/gnome/control-center/power">
     <file preprocess="xml-stripblanks">cc-power-panel.ui</file>
+    <file>battery-levels.css</file>
   </gresource>
 </gresources>