Style fixes.

frontend/app/theme/_bootstrap.scss

@@ -1,3 +1,6 @@
+@import '_mixins';
+@import '_vars';
+
 @import './../../node_modules/bootstrap/scss/mixins/_buttons';
 
 //

frontend/app/theme/_common.scss

@@ -1,3 +1,6 @@
+@import '_mixins';
+@import '_vars';
+
 body {
     // Default body with padding for fixed navbar and sidebar.
     & {

frontend/app/theme/_forms.scss

@@ -1,3 +1,6 @@
+@import '_mixins';
+@import '_vars';
+
 //
 // Support for Angular validation states.
 // 

frontend/app/theme/_lists.scss

@@ -1,3 +1,6 @@
+@import '_mixins';
+@import '_vars';
+
 //
 // Custom list that can either be used with a table or divs.
 //

frontend/app/theme/_mixins.scss

@@ -1,3 +1,5 @@
+// sass-lint:disable no-vendor-prefixes, one-declaration-per-line, no-misspelled-properties, no-color-keywords, no-color-literals
+
 @mixin build-icon($color) {
     & {
         color: $color;
@@ -71,23 +73,26 @@
     }
 }
 
-@mixin hidden() {
+@mixin hidden {
     opacity: 0;
-    overflow: hidden;
+    overflow-x: hidden;
+    overflow-y: hidden;
     position: absolute;
     width: 0;
 }
 
 @mixin force-width($width) {
-    width:  $width;
     // Ensure that we use the minimum width in flex scenarios.
     max-width: $width;
     min-width: $width;
+    // Normal width definition.
+    width:  $width;
 }
 
 @mixin force-height($height) {
+    // Normal height definition.
     height:  $height;
-    // Ensure that we use the minimum width in flex scenarios.
+    // Ensure that we use the minimum height in flex scenarios.
     max-height: $height;
     min-height: $height;
 }

frontend/app/theme/_panels.scss

@@ -1,3 +1,6 @@
+@import '_mixins';
+@import '_vars';
+
 @mixin panel-icon {
     color: $color-border-dark;
     cursor: pointer;

frontend/app/theme/_static.scss

@@ -1,3 +1,6 @@
+@import '_mixins';
+@import '_vars';
+
 //
 // Noscript element with warning, when javascript is disabled.
 //