Fix download-link color in dark boostrap themes

This makes the "Download attachment" link white in bootstrap-dark and
boostrap-dark-page. This is an upstream bug of darkstrap, but it's not
maintained anymore.

css/bootstrap/privatebin.css

@@ -144,3 +144,7 @@ footer h4 {
 li.L0, li.L1, li.L2, li.L3, li.L5, li.L6, li.L7, li.L8 {
 	list-style-type: decimal !important;
 }
+
+.dark-theme .alert-info .alert-link {
+	color: #fff;
+}

tpl/bootstrap.php

@@ -92,6 +92,9 @@ endif;
 if ($isCpct):
 ?> class="navbar-spacing"<?php
 endif;
+if ($isDark):
+?> class="dark-theme"<?php
+endif;
 ?>>
 		<div id="passwordmodal" tabindex="-1" class="modal fade" role="dialog" aria-hidden="true">
 			<div class="modal-dialog" role="document">