Themes: make 'description' and 'title' optional if there is a single theme and it can be reused from this single theme

src/Models/ThemeConfig/Conversion/PrepareTheme.ts

@@ -617,6 +617,33 @@ class OrderTheme extends DesugaringStep<ThemeConfigJson>{
         return Utils.reorder(json, OrderTheme.themeAttributesOrder)
     }
 }
+
+class DeriveDescription extends DesugaringStep<ThemeConfigJson> {
+    private readonly _key: string
+    private readonly _sourceKey: "name"
+    constructor(key: "icon" | "description" | "title", sourceKey?: "name") {
+        super("DeriveDescription", "If a single layer and no description is given, steal the description from the layer")
+        this._key = key
+        this._sourceKey = sourceKey
+    }
+
+    convert(json: ThemeConfigJson, context: ConversionContext): ThemeConfigJson {
+        if (json[this._key]) {
+            return json
+        }
+        const msg = "This theme has no "+this._key+". If there is only a single layer, we can steal it from this layer though. Current layers are "+json.layers.map(l => l["id"]).join("; ")
+        if (json.layers.length !== 1) {
+            context.err(msg)
+        }
+        const l = <LayerConfigJson> json.layers[0] // Already expanded
+        context.info(`Added '${this._key}' to theme ${json.id} based on single layer`)
+        return {
+            ...json,
+            [this._key]: l[this._sourceKey ?? this._key],
+        }
+    }
+}
+
 export class PrepareTheme extends Fuse<ThemeConfigJson> {
     private state: DesugaringContext
 
@@ -632,6 +659,9 @@ export class PrepareTheme extends Fuse<ThemeConfigJson> {
             new PreparePersonalTheme(state),
             new WarnForUnsubstitutedLayersInTheme(),
             new On("layers", new Concat(new SubstituteLayer(state))),
+            new DeriveDescription( "description"),
+            new DeriveDescription( "icon"),
+            new DeriveDescription( "title", "name"),
 
             new SetDefault("socialImage", "assets/SocialImage.png", true),
             // We expand all tagrenderings first...

src/Models/ThemeConfig/Conversion/ValidateTheme.ts

@@ -35,6 +35,9 @@ export class ValidateTheme extends DesugaringStep<ThemeConfigJson> {
     }
 
     convert(json: ThemeConfigJson, context: ConversionContext): ThemeConfigJson {
+        if (!json.title) {
+            context.enter("title").err(`The theme ${json.id} does not have a title defined.`)
+        }
         const theme = new ThemeConfig(json, this._isBuiltin)
         {
             // Legacy format checks
@@ -55,9 +58,7 @@ export class ValidateTheme extends DesugaringStep<ThemeConfigJson> {
                 }
             }
         }
-        if (!json.title) {
-            context.enter("title").err(`The theme ${json.id} does not have a title defined.`)
-        }
+
         if (!json.icon) {
             context.enter("icon").err("A theme should have an icon")
         }

src/Models/ThemeConfig/Json/ThemeConfigJson.ts

@@ -56,10 +56,11 @@ export interface ThemeConfigJson {
     /**
      * question: How would you describe this theme?
      * The description, as shown in the welcome message and the more-screen
+     * ifunset: reuse the description of the only layer
      * group: basic
      *
      */
-    description: Translatable
+    description?: Translatable
 
     /**
      * A short description, showed as social description and in the 'more theme'-buttons.

src/Models/ThemeConfig/ThemeConfig.ts

@@ -132,9 +132,7 @@ export default class ThemeConfig implements ThemeInformation {
         }
         const context = this.id
         this.credits = Array.isArray(json.credits) ? json.credits.join("; ") : json.credits
-        if (!json.title) {
-            throw `The theme ${json.id} does not have a title defined.`
-        }
+
         this.language = json.mustHaveLanguage ?? Object.keys(json.title)
 
         {