Implement overlay popup

client/css/app.css

@@ -533,3 +533,27 @@ span.tag {
     margin-right: 0.5em;
   }
 }
+
+.overlay-wrapper {
+  position: fixed;
+  left: 0;
+  right: 0;
+  top: 0;
+  bottom: 0;
+  background-color: rgba(0, 0, 0, 0.2);
+  z-index: 100;
+  transition: opacity var(--fade), top var(--fade);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+
+  & > .overlay {
+    max-width: 90%;
+    max-height: 90%;
+    background-color: white;
+    border: 1px solid var(--color1);
+    border-radius: 10px;
+    padding: 10px;
+    box-shadow: 0 0 15px #aaa;
+  }
+}