added threma to brands.css, their svg and to index.html

css/brands.css

@@ -578,3 +578,13 @@ button:hover,
 .button.button-yt:focus {
   filter: brightness(90%);
 }
+
+/* Threema */
+.button.button-threema {
+  color: #000000;
+  background-color: #3fe669;
+}
+.button.button-threema:hover,
+.button.button-threema:focus {
+  filter: brightness(90%);
+}