panel : mixer : restore flat style for buttons

it was accidentally removed in 77c16f9

Author: AKArien

src/panel/widgets/mixer/mixer-control.cpp

@@ -32,6 +32,7 @@ void MixerControl::init()
     add_css_class("mixer-control");
     button.set_child(volume_icon);
     button.add_css_class("mute-toggle");
+    button.add_css_class("flat");
     volume_icon.add_css_class("default-icon");
 
     scale.set_range(0.0, 1.0);
@@ -224,6 +225,7 @@ MixerControlDevice::~MixerControlDevice()
 void MixerControlDevice::init()
 {
     add_css_class("mixer-control");
+    default_btn.add_css_class("flat");
     default_btn.add_css_class("default-button");
 
     is_def_icon.set_from_icon_name("emblem-default");