fix: invalid scss variable in css function

This issue was caused by the PR.

Author: jeffreytse

_sass/misc/article-menu.scss

@@ -16,7 +16,7 @@
   .post-menu-content {
     ul {
       border-left: 1px solid #e9ecef;
-      $indent: calc($base-font-size / 4);
+      $indent: #{$base-font-size / 4};
       $active-bgcolor: #ecebec;
 
       @for $i from 2 to 7 {

_sass/yat.scss

@@ -43,8 +43,8 @@ $on-laptop:        800px !default;
 // Use media queries like this:
 // @include media-query($on-palm) {
 //   .wrapper {
-//     padding-right: calc($spacing-unit / 2);
-//     padding-left: calc($spacing-unit / 2);
+//     padding-right: #{$spacing-unit / 2};
+//     padding-left: #{$spacing-unit / 2};
 //   }
 // }
 @mixin media-query($device) {

_sass/yat/_base.scss

@@ -42,7 +42,7 @@ h1, h2, h3, h4, h5, h6,
 p, blockquote, pre,
 ul, ol, dl, figure,
 %vertical-rhythm {
-  margin-bottom: calc($spacing-unit / 2);
+  margin-bottom: #{$spacing-unit / 2};
 }
 
 
@@ -128,7 +128,7 @@ a {
 blockquote {
   color: $grey-color;
   border-left: 4px solid $grey-color-light;
-  padding-left: calc($spacing-unit / 2);
+  padding-left: #{$spacing-unit / 2};
   @include relative-font-size(1.125);
   letter-spacing: -1px;
   font-style: italic;
@@ -222,7 +222,7 @@ table {
   }
 
   th, td {
-    padding: calc($spacing-unit / 3) calc($spacing-unit / 2);
+    padding: #{$spacing-unit / 3} #{$spacing-unit / 2};
   }
 
   th {

_sass/yat/_layout.scss

@@ -107,7 +107,7 @@ html {
 
     .site-favicon {
       display: inline-block;
-      height: calc($header-height / 1.5);
+      height: #{$header-height / 1.5};
       margin-right: 5px;
     }
   }
@@ -695,7 +695,7 @@ html {
   }
 
   @include media-query($on-palm) {
-    height: calc($banner-height / 1.5);
+    height: #{$banner-height / 1.5};
   }
 }