Darken input border. Darken more on hover.

scss/_variables.scss

@@ -82,7 +82,8 @@ $code-color: $primary !default;
 // Forms
 $input-bg: $white;
 $input-color: $black;
-$input-transition: null;
+$input-border-color: $gray-500;
+$input-transition: none;
 
 // Buttons
 $btn-box-shadow: null;

scss/bootstrap/_form-control.scss

@@ -1,3 +1,14 @@
 .form-control {
   background-clip: inherit; // Remove weird border in input boxes
 }
+
+.form-control,
+.form-select {
+  &:hover {
+    border-color: $gray-600;
+  }
+
+  &:focus {
+    border-color: $input-focus-border-color;
+  }
+}