week-header: adjust theming to match the mockups

Set correct font weight, size and draw lines where required.

data/ui/week-header.ui

@@ -3,6 +3,9 @@
   <template class="GcalWeekHeader" parent="GtkGrid">
     <property name="visible">True</property>
     <property name="hexpand">True</property>
+    <style>
+      <class name="week-header" />
+    </style>
     <child>
       <object class="GtkGrid">
         <property name="visible">True</property>
@@ -16,7 +19,7 @@
                 <property name="visible">True</property>
                 <property name="label">Month</property>
                 <style>
-                  <class name="first-view-header" />
+                  <class name="month-name" />
                 </style>
               </object>
             </child>
@@ -27,7 +30,7 @@
                 <property name="xalign">0</property>
                 <property name="label">Week</property>
                 <style>
-                  <class name="second-view-header" />
+                  <class name="week-number" />
                 </style>
               </object>
             </child>
@@ -36,7 +39,7 @@
                 <property name="visible">True</property>
                 <property name="label">Year</property>
                 <style>
-                  <class name="second-view-header" />
+                  <class name="week-number" />
                 </style>
               </object>
             </child>
@@ -50,6 +53,9 @@
                 <property name="visible">True</property>
                 <property name="height-request">100</property>
                 <signal name="draw" handler="gcal_week_header_draw" object="GcalWeekHeader" swapped="yes"/>
+                <style>
+                  <class name="week-header" />
+                </style>
               </object>
             </child>
             <child type="overlay">