style: define figucaption and img in figure

Author: yoshinorin

src/styles/globals.scss

@@ -252,5 +252,15 @@ figure {
     width:  auto;
     margin: 0;
   }
+  figcaption {
+    margin-top: 0.9em;
+    text-align: center;
+    font-size: map.get(variables.$font-size, normal);
+    opacity: 0.7;
+    line-height: 1rem;
+  }
+  img {
+    border-radius: 0.3em;
+  }
 }