css: fix styling of selected post in parent/child box.

* On the post show page, in the parent/child box, change it so that
  currently selected post is more visible. Use a light blue outline
  around the current post instead of a dark semi-transparent border.

* Fix it so that there's less empty space around thumbnails in the
  parent/child box.

app/components/post_preview_component/post_preview_component.scss

@@ -122,10 +122,6 @@ body[data-current-user-can-approve-posts="true"] .post-preview {
   }
 }
 
-.post-preview.current-post {
-  background-color: var(--preview-current-post-background);
-}
-
 @media screen and (max-width: 660px) {
   article.post-preview {
     margin: 0;

app/javascript/src/styles/base/040_colors.css

@@ -144,6 +144,7 @@ html {
   --preview-deleted-color: var(--grey-9);
   --preview-has-children-color: var(--green-3);
   --preview-has-parent-color: var(--orange-3);
+  --preview-selected-color: var(--blue-3);
 
   --preview-icon-color: var(--inverse-text-color);
   --preview-icon-background: rgba(0, 0, 0, 0.5);
@@ -181,8 +182,6 @@ html {
   --user-tooltip-positive-feedback-color: var(--orange-3);
   --user-tooltip-negative-feedback-color: var(--red-5);
 
-  --preview-current-post-background: rgba(0, 0, 0, 0.1);
-
   --autocomplete-selected-background-color: var(--blue-0);
   --autocomplete-border-color: var(--grey-2);
   --autocomplete-tag-autocorrect-underline: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAQAAAADCAYAAAC09K7GAAAAHElEQVQYV2NkQAL/GRj+M4IJBgY4zQhSABMEsQHMOAgCT5YN9gAAAABJRU5ErkJggg==);
@@ -357,6 +356,7 @@ body[data-current-user-theme="dark"] {
   --preview-deleted-color: var(--grey-3);
   --preview-has-children-color: var(--green-3);
   --preview-has-parent-color: var(--orange-3);
+  --preview-selected-color: var(--blue-3);
 
   --modqueue-tag-warning-color: var(--red-7);
 

app/javascript/src/styles/specific/posts.scss

@@ -37,10 +37,15 @@
 
   article.post-preview {
     width: auto;
-    border: none;
-    margin: 0;
-    padding: 5px 5px 10px;
-    max-width: 154px;
+    height: auto;
+    margin: 0.5rem 0.25rem;
+    padding: 0.25rem;
+    overflow: hidden;
+    vertical-align: top;
+
+    &.current-post {
+      outline: 3px solid var(--preview-selected-color);
+    }
 
     img {
       white-space: normal;