Improve modal css

assets/sass/_form.sass

@@ -119,6 +119,7 @@ ul.form-errors li
 
   .form-column
     margin-right: 25px
+    flex-grow: 1
 
 .form-login
   max-width: 350px

assets/sass/_popover.sass

@@ -7,27 +7,21 @@
   left: 0
   width: 100%
   height: 100%
-  background: rgba(0, 0, 0, 0.8)
+  background: rgba(0, 0, 0, 0.9)
   overflow: auto
   z-index: 100
 
 #popover-content
-  position: absolute
-  width: 75%
-  left: 12%
-  top: 1%
+  position: fixed
+  width: 950px
+  max-width: 95%
+  max-height: calc(100% - 100px)
+  top: 5%
+  left: 50%
+  transform: translate(-50%, -5%)
   padding: 15px
   background: #fff
   overflow: auto
-  max-height: 95%
-
-  @include xs-device
-    left: 0
-    width: 100%
-
-  @include sm-device
-    left: 2.5%
-    width: 85%
 
 .popover-form
   margin-bottom: 0