Make mixins use font mixins

src/styles/mixins/forms.scss

@@ -50,6 +50,7 @@
 
 @mixin primary-action-btn {
   @include focus.outline-on-keyboard-focus;
+  @include var.font-normal;
   color: var.$grey-1;
   background-color: var.$grey-mid;
   height: 35px;
@@ -57,7 +58,6 @@
   border-radius: 2px;
 
   font-weight: bold;
-  font-size: var.$body1-font-size;
 
   padding-left: 12px;
   padding-right: 12px;

src/styles/mixins/panel.scss

@@ -25,8 +25,8 @@
   }
 
   &__title {
+    @include var.font-big;
     color: var.$color-brand;
-    font-size: var.$body2-font-size;
     font-weight: 700;
   }