fix(editor): adjust TOC colors for accessability

Previously the hardcoded colors only made a contrast of 3.39 to the
background. Furthermore changing the overall theme variables wouldn't
change the colors for the table-of-contents as well. This commit
changes that to use the theme variables, therefore ensuring better
maintainability and contrast (now 7.29)

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

frontend/src/components/editor-page/table-of-contents/table-of-contents.module.scss

@@ -43,7 +43,7 @@
     font-size: 0.9em;
 
     a {
-      color: #767676;
+      color: var(--bs-secondary-color);
       overflow: hidden;
       text-overflow: ellipsis;
       white-space: pre;
@@ -54,7 +54,7 @@
       a {
         margin-top: 2px;
         margin-bottom: 2px;
-        color: #000000;
+        color: var(--bs-emphasis-color);
         font-weight: bold;
       }
     }