Fix custom theme color setting in config panel (#1268)

* Properly update the theme header with custom theme color values 
* Closes #1263

CTFd/themes/admin/assets/js/pages/configs.js

@@ -245,18 +245,20 @@ $(() => {
   $(".nav-pills a").click(showTab);
   $("#config-color-update").click(function() {
     const hex_code = $("#config-color-picker").val();
-    const user_css = $("#css-editor").val();
+    const user_css = $("#theme-header").val();
     let new_css;
-    if (user_css.lenth) {
+    if (user_css.length) {
       let css_vars = `theme-color: ${hex_code};`;
       new_css = user_css.replace(/theme-color: (.*);/, css_vars);
     } else {
       new_css =
+        `<style id="theme-color">\n` +
         `:root {--theme-color: ${hex_code};}\n` +
         `.navbar{background-color: var(--theme-color) !important;}\n` +
-        `.jumbotron{background-color: var(--theme-color) !important;}\n`;
+        `.jumbotron{background-color: var(--theme-color) !important;}\n` +
+        `</style>\n`;
     }
-    $("#css-editor").val(new_css);
+    $("#theme-header").val(new_css);
   });
 
   $(".start-date").change(function() {