fixed is-url & is-background-image issue

scss/mixins/_background.scss

@@ -1,7 +1,7 @@
 $background-image-modes: array-append((none), $css-default-modes);
 
 @function is-background-image($bi) {
-  @return contains($background-image-modes, $bi) or is-url($bi);
+  @return is-url($bi) or contains($background-image-modes, $bi);
 }
 
 $background-position-horizontal-modes: (left, center, right);

scss/mixins/_functions.scss

@@ -72,7 +72,7 @@ $color-modes: array-append((transparent), $css-default-modes);
     $h-length: str-length($h);
     $n-length: str-length($n);
 
-    @return str-slice($h, $h-length - $n-length, $n-length)==$n;
+    @return str-slice($h, $h-length - $n-length + 1)==$n;
   }
 
   @warn '`haystack: #{$h}` and `needle: #{$n}` are not valid strings';
@@ -83,15 +83,19 @@ $color-modes: array-append((transparent), $css-default-modes);
   @if is-string($u) {
     $url: $u;
 
-    @if string-starts-with($u, 'url(') and string-ends-with($u, ')') {
-    //   $url: str-slice($u, 5, str-length($u) - 1);
-    // }
+    $urlStartsWith: string-starts-with($url, 'url("');
+    $urlEndsWith: string-ends-with($url, '")');
 
-    // @if string-starts-with($url, '../') or string-starts-with($url, '/') or string-starts-with($url, 'http') {
+    @if (string-starts-with($url, 'url("') and string-ends-with($url, '")')) or (string-starts-with($url, 'url(\'') and string-ends-with($url, '\')')) {
+      $url: str-slice($u, 6, str-length($u) - 2);
+    }
+
+    @if string-starts-with($url, '../') or string-starts-with($url, '/') or string-starts-with($url, 'http') {
       @return true;
     }
   }
 
+  @warn '`url: #{$u}` is not a valid string';
   @return false;
 }