Fixed observatory header buttons font

Fixes=#27194

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Review URL: https://codereview.chromium.org/2296453002 .

Author: B3rn475

runtime/observatory/lib/src/elements/css/shared.css

@@ -11,6 +11,10 @@ body {
   font: 400 14px 'Montserrat', sans-serif;
 }
 
+button {
+  font: 400 14px 'Montserrat', sans-serif;
+}
+
 .content {
   padding-left: 10%;
   font: 400 14px 'Montserrat', sans-serif;