fix hf css

src/main/ui/static/css/hf_gateway.css

@@ -10,6 +10,11 @@
     z-index: -1;
     overflow-y: auto;
     background-size:100% 100%;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    align-content: center;
+    flex-direction: column;
 }
 
 .head-bar{
@@ -132,3 +137,15 @@
         padding:50px 10px;
     }
 }
+
+.hf-gateway-background .modal {
+    background-color: rgba(0,0,0,.3);
+}
+
+.hf-gateway-background .modal-dialog {
+    height: 100vh;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    align-content: center;
+}