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

index.html

@@ -370,6 +370,11 @@
             <img class="icon" src="images/icons/vrchat.svg" alt="VRChat Logo">VRChat</a>
         <br>
 
+        <!-- Threema -->
+        <a class="button button-threema" href="#" target="_blank" rel="noopener">
+            <img class="icon" src="images/icons/threema.svg" alt="Threema Logo">Threema</a>
+        <br>
+
         <br>
         <p>🔒
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