Use btn-dark everywhere. Closes BiscuIT-ng#94.

biscuit/apps/untis/templates/untis/untis_import.html

@@ -37,7 +37,7 @@
     {% csrf_token %}
     {% bootstrap_form upload_form %}
 
-    <button type="submit" class="btn btn-secondary">
+    <button type="submit" class="btn btn-dark">
       {% blocktrans %}Process{% endblocktrans %}
     </button>
   </form>