Add custom CSS setting for admin customization

- Add custom_css textarea setting in settings.yml
- Support custom CSS injection in welcome-banner component
- Allow admins to override or extend banner styles without modifying code
- Inject custom CSS only when banner is displayed
- Include English and Hungarian descriptions with examples

javascripts/discourse/components/welcome-banner.gjs

@@ -47,6 +47,9 @@ export default class WelcomeBanner extends Component {
 
   <template>
     {{#if this.shouldDisplay}}
+      {{#if settings.custom_css}}
+        <style>{{settings.custom_css}}</style>
+      {{/if}}
       <section class={{concat settings.banner_position "-outlet"}} {{didInsert this.didInsert}} {{willDestroy this.willDestroy}}>
         <div class="welcome-card">
           <div class="hero-section">

settings.yml

@@ -329,3 +329,12 @@ cta_card_4_url:
   description:
     en: CTA card 4 URL
     hu: 4. CTA kártya URL-je
+
+# Advanced Customization
+custom_css:
+  default: ""
+  type: string
+  textarea: true
+  description:
+    en: "Custom CSS to override or extend banner styles. Example: .welcome-card { background: blue; }"
+    hu: "Egyedi CSS a banner stílusok felülírásához vagy kiterjesztéséhez. Példa: .welcome-card { background: blue; }"