Introduce Dark Mode

This commit mainly changes the color definitions. Mostly, those changes are semantically equally, but there are a few changes that occurred to align the color scheme within the app.

app/views/request_for_comments/_list_entry.html.slim

@@ -3,7 +3,7 @@ tr.table-row-clickable data-id=request_for_comment.id data-href=request_for_comm
     - if request_for_comment.solved?
       span.fa-solid.fa-check.fa-2x.text-success aria-hidden="true"
     - elsif request_for_comment.full_score_reached
-      span.fa-solid.fa-check.fa-2x style="color:darkgrey" aria-hidden="true"
+      span.fa-solid.fa-check.fa-2x style="color: var(--bs-secondary-text-emphasis);" aria-hidden="true"
     - else
       = ''
   td.text-center = request_for_comment.comments_count

app/views/request_for_comments/index.html.slim

@@ -37,7 +37,7 @@ h1 = RequestForComment.model_name.human(count: 2)
               span class="fa-solid fa-check" aria-hidden="true"
           - elsif request_for_comment.full_score_reached
             td
-              span class="fa-solid fa-check" style="color:darkgrey" aria-hidden="true"
+              span class="fa-solid fa-check" style="color: var(--bs-secondary-text-emphasis);" aria-hidden="true"
           - else
             td = ''
           td = link_to_if(policy(request_for_comment).show?, request_for_comment.submission.exercise.title, request_for_comment)

app/views/request_for_comments/show.html.slim

@@ -1,7 +1,7 @@
 .list-group
   h4#exercise_caption.list-group-item-heading data-exercise-id="#{@request_for_comment.exercise.id}" data-rfc-id="#{@request_for_comment.id}"
     - if @request_for_comment.solved?
-      span.fa-solid.fa-check aria-hidden="true"
+      span.fa-solid.fa-check.me-2 aria-hidden="true"
     = link_to_if(policy(@request_for_comment.exercise).show?, @request_for_comment.exercise.title, [:implement, @request_for_comment.exercise])
   p.list-group-item-text
     - user = @request_for_comment.user