✨ Remove unused legacy sass files

frontend/resources/styles/common/base.scss

@@ -5,6 +5,7 @@
 // Copyright (c) KALEIDOS INC
 
 // TODO: Legacy sass vars. We should use DS tokens.
+$color-gray-50: #303236;
 $fs12: 0.75rem;
 $fs18: 1.125rem;
 $fs24: 1.5rem;

frontend/resources/styles/common/dependencies/highlightjs-theme.scss

@@ -1,84 +0,0 @@
-/*
-Monokai Sublime style. Derived from Monokai by noformnocontent http://nn.mit-license.org/
-*/
-
-// TODO: Legacy sass vars. We should use DS tokens.
-$fw700: 700;
-
-.hljs {
-  display: block;
-  overflow-x: auto;
-  padding: 0.5em;
-  background: #23241f;
-}
-
-.hljs,
-.hljs-tag,
-.hljs-subst {
-  color: #f8f8f2;
-}
-
-.hljs-strong,
-.hljs-emphasis {
-  color: #a8a8a2;
-}
-
-.hljs-bullet,
-.hljs-quote,
-.hljs-number,
-.hljs-regexp,
-.hljs-literal,
-.hljs-link {
-  color: #ae81ff;
-}
-
-.hljs-code,
-.hljs-title,
-.hljs-section,
-.hljs-selector-class {
-  color: #a6e22e;
-}
-
-.hljs-strong {
-  font-weight: $fw700;
-}
-
-.hljs-emphasis {
-  font-style: italic;
-}
-
-.hljs-keyword,
-.hljs-selector-tag,
-.hljs-name,
-.hljs-attr {
-  color: #f92672;
-}
-
-.hljs-symbol,
-.hljs-attribute {
-  color: #66d9ef;
-}
-
-.hljs-params,
-.hljs-class .hljs-title {
-  color: #f8f8f2;
-}
-
-.hljs-string,
-.hljs-type,
-.hljs-built_in,
-.hljs-builtin-name,
-.hljs-selector-id,
-.hljs-selector-attr,
-.hljs-selector-pseudo,
-.hljs-addition,
-.hljs-variable,
-.hljs-template-variable {
-  color: #e6db74;
-}
-
-.hljs-comment,
-.hljs-deletion,
-.hljs-meta {
-  color: #75715e;
-}

frontend/resources/styles/common/dependencies/z-index.scss

@@ -1,10 +0,0 @@
-// This Source Code Form is subject to the terms of the Mozilla Public
-// License, v. 2.0. If a copy of the MPL was not distributed with this
-// file, You can obtain one at http://mozilla.org/MPL/2.0/.
-//
-// Copyright (c) KALEIDOS INC
-
-$autocomplete: 30000;
-$index-lightbox-shadow: 60000;
-$index-lightbox: 60001;
-$index-lightbox-close-x: 200;

frontend/resources/styles/common/refactor/color-defs.scss

@@ -9,7 +9,6 @@
 // TODO: legacy Sass colors, we should eventually try to remove them in
 //       favor of DS colors.
 $color-gray-20: #b1b2b5;
-$color-gray-50: #303236;
 
 :root {
   // DARK