Dedupe some SCSS variables

Use the corresponding color and code instead to make
switching color schemes easier

app/assets/stylesheets/framework/buttons.scss

@@ -120,7 +120,7 @@
 }
 
 @mixin btn-white {
-  @include btn-color($white, $border-color, $white-normal, $border-white-normal, $white-dark, $border-gray-dark, $gl-text-color);
+  @include btn-color($white, $border-color, $white-normal, $border-white-normal, $white-dark, $border-white-normal, $gl-text-color);
 }
 
 @mixin btn-with-margin {
@@ -365,7 +365,7 @@
   .active {
     box-shadow: $gl-btn-active-background;
 
-    border: 1px solid $border-gray-dark !important;
+    border: 1px solid $border-white-normal !important;
     background-color: $btn-active-gray-light !important;
   }
 }

app/assets/stylesheets/framework/variables.scss

@@ -329,7 +329,6 @@ $border-white-normal: darken($white-normal, $darken-border-factor);
 $border-gray-light: darken($gray-light, $darken-border-factor);
 $border-gray-normal: darken($gray-normal, $darken-border-factor);
 $border-gray-normal-dashed: darken($gray-normal, $darken-border-dashed-factor);
-$border-gray-dark: darken($white-normal, $darken-border-factor);
 
 /*
  * UI elements
@@ -350,13 +349,13 @@ $gl-font-size-small: 12px;
 $gl-font-size-large: 16px;
 $gl-font-weight-normal: 400;
 $gl-font-weight-bold: 600;
-$gl-text-color: #2e2e2e;
-$gl-text-color-secondary: #707070;
-$gl-text-color-tertiary: #919191;
+$gl-text-color: $gray-900;
+$gl-text-color-secondary: $gray-700;
+$gl-text-color-tertiary: $gray-600;
 $gl-text-color-quaternary: #d6d6d6;
-$gl-text-color-inverted: rgba(255, 255, 255, 1);
+$gl-text-color-inverted: $white;
 $gl-text-color-secondary-inverted: rgba(255, 255, 255, 0.85);
-$gl-text-color-disabled: #919191;
+$gl-text-color-disabled: $gray-600;
 $gl-grayish-blue: #7f8fa4;
 $gl-gray-dark: #313236;
 $gl-gray-light: #5c5c5c;

app/assets/stylesheets/pages/boards.scss

@@ -475,7 +475,7 @@
   }
 
   .board-card {
-    border: 1px solid $border-gray-dark;
+    border: 1px solid $border-white-normal;
     box-shadow: 0 1px 2px rgba($issue-boards-card-shadow, 0.3);
     cursor: pointer;
   }

app/assets/stylesheets/pages/commits.scss

@@ -131,7 +131,7 @@
   color: $gl-text-color-secondary;
   padding: 1px $gl-padding-4;
   cursor: pointer;
-  border: 1px solid $border-gray-dark;
+  border: 1px solid $border-white-normal;
   border-radius: $border-radius-default;
   margin-left: 5px;
   font-size: 12px;

app/assets/stylesheets/pages/issuable.scss

@@ -432,7 +432,7 @@
       height: $sidebar-toggle-height;
       margin-left: 0;
       padding-left: 0;
-      border-bottom: 1px solid $border-gray-dark;
+      border-bottom: 1px solid $border-white-normal;
     }
 
     a.gutter-toggle {

app/assets/stylesheets/pages/tree.scss

@@ -129,8 +129,8 @@
       &.selected {
         td {
           background: $white-normal;
-          border-top: 1px solid $border-gray-dark;
-          border-bottom: 1px solid $border-gray-dark;
+          border-top: 1px solid $border-white-normal;
+          border-bottom: 1px solid $border-white-normal;
         }
       }
     }