css/themes: add a Winter theme

fixes #10898

public/css/themes/Winter.less

@@ -0,0 +1,38 @@
+@icinga-blue: #001E5b;
+
+#icinga-logo {
+    background-image: url('../img/winter/logo_icinga_big_winter.png');
+}
+
+@tr-active-color: #dadeea;
+@tr-hover-color: #c6cfe4;
+
+/* Snow, from http://codepen.io/NickyCDK/pen/AIonk */
+#header, #login {
+    background-image: url('../img/winter/snow1.png'), url('../img/winter/snow2.png'), url('../img/winter/snow3.png');
+    -webkit-animation: snow 10s linear infinite;
+    -moz-animation: snow 10s linear infinite;
+    -ms-animation: snow 10s linear infinite;
+    animation: snow 10s linear infinite;
+}
+
+@keyframes snow {
+  0% {background-position: 0px 0px, 0px 0px, 0px 0px;}
+  50% {background-position: 500px 500px, 100px 200px, -100px 150px;}
+  100% {background-position: 500px 1000px, 200px 400px, -100px 300px;}
+}
+@-moz-keyframes snow {
+  0% {background-position: 0px 0px, 0px 0px, 0px 0px;}
+  50% {background-position: 500px 500px, 100px 200px, -100px 150px;}
+  100% {background-position: 400px 1000px, 200px 400px, 100px 300px;}
+}
+@-webkit-keyframes snow {
+  0% {background-position: 0px 0px, 0px 0px, 0px 0px;}
+  50% {background-position: 500px 500px, 100px 200px, -100px 150px;}
+  100% {background-position: 500px 1000px, 200px 400px, -100px 300px;}
+}
+@-ms-keyframes snow {
+  0% {background-position: 0px 0px, 0px 0px, 0px 0px;}
+  50% {background-position: 500px 500px, 100px 200px, -100px 150px;}
+  100% {background-position: 500px 1000px, 200px 400px, -100px 300px;}
+}