Update X share button to match brand color (#5245)

Author: y-kawa

_sass/minimal-mistakes/_buttons.scss

@@ -43,7 +43,7 @@
   (linkedin, $linkedin-color),
   (mastodon, $mastodon-color),
   (twitter, $twitter-color),
-  (x, $twitter-color);
+  (x, $x-color);
 
   @each $buttoncolor, $color in $buttoncolors {
     &--#{$buttoncolor} {

_sass/minimal-mistakes/_utilities.scss

@@ -218,7 +218,8 @@ $text-alignments: left, right, start, end, center, justify;
     $soundcloud-color: ".fa-soundcloud",
     $stackoverflow-color: ".fa-stack-exchange, .fa-stack-overflow",
     $tumblr-color: ".fa-tumblr, .fa-tumblr-square",
-    $twitter-color: ".fa-twitter, .fa-twitter-square, .fa-x-twitter",
+    $twitter-color: ".fa-twitter, .fa-twitter-square",
+    $x-color: ".fa-x-twitter, .fa-square-x-twitter",
     $vimeo-color: ".fa-vimeo, .fa-vimeo-square, .fa-vimeo-v",
     $vine-color: ".fa-vine",
     $xing-color: ".fa-xing, .fa-xing-square",

_sass/minimal-mistakes/_variables.scss

@@ -142,6 +142,7 @@ $soundcloud-color: #ff3300 !default;
 $stackoverflow-color: #fe7a15 !default;
 $tumblr-color: #32506d !default;
 $twitter-color: #55acee !default;
+$x-color: #0f1419 !default;
 $vimeo-color: #1ab7ea !default;
 $vine-color: #00bf8f !default;
 $youtube-color: #bb0000 !default;