Fix missing serif variable in magnfic popup styles (#295)

Author: DiogoRibeiro7

_sass/minimal-mistakes/vendor/magnific-popup/_magnific-popup.scss

@@ -292,7 +292,7 @@ button {
 
   font-style: normal;
   font-size: 28px;
-  font-family: $serif;
+  font-family: $caption-font-family;
 
   &:hover,
   &:focus {

_sass/minimal-mistakes/vendor/magnific-popup/_settings.scss

@@ -42,7 +42,7 @@ $mfp-include-mobile-layout-for-image: true;                       // Removes pad
 // Image caption options
 $mfp-caption-title-color:             #f3f3f3;                    // Caption title color
 $mfp-caption-subtitle-color:          #bdbdbd;                    // Caption subtitle color
-.mfp-counter { font-family: $serif; }                          // Caption font family
+.mfp-counter { font-family: $caption-font-family; }                          // Caption font family
 
 // A11y
 $mfp-use-visuallyhidden:              false;