Show thumbnails as tooltips.

src/scss/thumbnail.scss

@@ -16,6 +16,10 @@
   border: 1px solid var(--primary-color);
   border-radius: var(--ha-card-border-radius, 4px);
   padding: 2px;
+
+  // When details are enabled, use a background color so that the details have
+  // contrast with the background.
+  background-color: var(--primary-background-color, black);
 }
 
 img {

src/scss/timeline-core.scss

@@ -85,4 +85,7 @@ div.vis-tooltip {
   padding: 0px;
   background-color: unset;
   border: none;
+
+  // Use browser default font-family for tooltips.
+  font-family: unset;
 }