Substitute deprecated ui_charcoal with new default ui_indigo

app/views/layouts/devise.html.haml

 !!! 5
 %html.devise-layout-html
   = render "layouts/head"
-  %body.ui_charcoal.login-page.application.navless{ data: { page: body_data_page } }
+  %body.ui_indigo.login-page.application.navless{ data: { page: body_data_page } }
     .page-wrap
       = render "layouts/header/empty"
       .login-page-broadcast

app/views/layouts/devise_empty.html.haml

 !!! 5
 %html{ lang: "en" }
   = render "layouts/head"
-  %body.ui_charcoal.login-page.application.navless
+  %body.ui_indigo.login-page.application.navless
     = render "layouts/header/empty"
     = render "layouts/broadcast"
     .container.navless-container

changelogs/unreleased/41744-substitute-ui-charcoal-with-ui-indigo.yml

+---
+title: Substitute deprecated ui_charcoal with new default ui_indigo
+merge_request: 16271
+author: Takuya Noguchi
+type: fixed

doc/development/fe_guide/style_guide_scss.md

@@ -216,7 +216,7 @@ If you want a line or set of lines to be ignored by the linter, you can use
 ```scss
 // This lint rule is disabled because the class name comes from a gem.
 // scss-lint:disable SelectorFormat
-.ui_charcoal {
+.ui_indigo {
   background-color: #333;
 }
 // scss-lint:enable SelectorFormat