Fix for mdl3.10+

The $brand-primary var was inherited from boost but doesn't exist anymore in the parent theme.
Using $primary fix the issue and add consistency within the subtheme

scss/style_post_scss.scss

@@ -362,7 +362,7 @@ body {
 
     .login-button {
       color: white !important;
-      background-color: $brand-primary;
+      background-color: $primary;
       margin-top: 0.60em;
       margin-bottom: 0.60em;
       margin-right: 0.4em;
@@ -374,7 +374,7 @@ body {
     .login-button:hover, .login-button:focus {
       color: white;
       text-decoration: none;
-      background-color: $brand-primary;
+      background-color: $primary;
     }
 
     .login-menu {
@@ -556,8 +556,8 @@ select + .btn-group, .btn-group + .btn  {
   }
 
   #enrolusersbutton-1 .btn {
-    color: $brand-primary;
-    border-color: $brand-primary;
+    color: $primary;
+    border-color: $primary;
     background-color: #fff;
     margin-top: 2.7em !important;
     transition: background-color 0.2s;
@@ -634,7 +634,7 @@ select + .btn-group, .btn-group + .btn  {
       display: block;
       height: 50px;
       color: #fff;
-      background-color: $brand-primary;
+      background-color: $primary;
       padding-top: 0.55em;
       padding-left: 0.9em;
       padding-right: 0.9em;
@@ -766,7 +766,7 @@ select + .btn-group, .btn-group + .btn  {
 
     .coursename {
       font-size: 1.17rem;
-      color: $brand-primary;
+      color: $primary;
     }
 
     .coursehidden {
@@ -907,14 +907,14 @@ select + .btn-group, .btn-group + .btn  {
           border-radius: .2em;
           background-color: white;
           border: 1px solid rgba(0,0,0,.15);
-          border-bottom-color: $brand-primary;
+          border-bottom-color: $primary;
           border-bottom-width: 0.2em;
           box-shadow: 0 2px 5px rgba(0,0,0,.05);
           transition: border 0.2s, background-color 0.2s;
         }
 
         a:hover, a:focus {
-          color: $brand-primary;
+          color: $primary;
           background-color: #fbfbfb;
           text-decoration: none;
           border-bottom-width: 0.4em;
@@ -1027,7 +1027,7 @@ select + .btn-group, .btn-group + .btn  {
     width: 4em;
     height: 4em;
     border-radius: 100%;
-    background-color: $brand-primary;
+    background-color: $primary;
     padding-top: 0.88em;
     position: fixed;
     right: 1.5em;
@@ -1049,7 +1049,7 @@ select + .btn-group, .btn-group + .btn  {
 
   .scroll-top-button:hover {
     padding-top: 0.35em;
-    background-color: $brand-primary;
+    background-color: $primary;
   }
 
   .footer-container {