Fix: (.image-grid > div) Image background and border color

For the "Visualization" tab.  In light mode, this color is slightly
different, but not noticeably unless seen together with the old color.
In dark mode it appears perfect.

Author: alphapapa

assets/css/tabs.scss

@@ -73,8 +73,8 @@ table td, table th {
 }
 
 .image-grid > div {
-    background-color: rgb(238, 238, 238);
-    border: 2px solid rgb(255, 255, 255);
+    background-color: var(--pst-color-surface);
+    border: 2px solid var(--pst-color-surface);
     border-radius: 10px;
     padding: 10px;
 }