diff --git a/Logic/DetermineLayout.ts b/Logic/DetermineLayout.ts
index 85da45e2bb..206c8e4c1c 100644
--- a/Logic/DetermineLayout.ts
+++ b/Logic/DetermineLayout.ts
@@ -1,23 +1,24 @@
 import LayoutConfig from "../Models/ThemeConfig/LayoutConfig"
-import { QueryParameters } from "./Web/QueryParameters"
-import { AllKnownLayouts } from "../Customizations/AllKnownLayouts"
-import { FixedUiElement } from "../UI/Base/FixedUiElement"
-import { Utils } from "../Utils"
+import {QueryParameters} from "./Web/QueryParameters"
+import {AllKnownLayouts} from "../Customizations/AllKnownLayouts"
+import {FixedUiElement} from "../UI/Base/FixedUiElement"
+import {Utils} from "../Utils"
 import Combine from "../UI/Base/Combine"
-import { SubtleButton } from "../UI/Base/SubtleButton"
+import {SubtleButton} from "../UI/Base/SubtleButton"
 import BaseUIElement from "../UI/BaseUIElement"
-import { UIEventSource } from "./UIEventSource"
-import { LocalStorageSource } from "./Web/LocalStorageSource"
+import {UIEventSource} from "./UIEventSource"
+import {LocalStorageSource} from "./Web/LocalStorageSource"
 import LZString from "lz-string"
-import { FixLegacyTheme } from "../Models/ThemeConfig/Conversion/LegacyJsonConvert"
-import { LayerConfigJson } from "../Models/ThemeConfig/Json/LayerConfigJson"
+import {FixLegacyTheme} from "../Models/ThemeConfig/Conversion/LegacyJsonConvert"
+import {LayerConfigJson} from "../Models/ThemeConfig/Json/LayerConfigJson"
 import SharedTagRenderings from "../Customizations/SharedTagRenderings"
 import * as known_layers from "../assets/generated/known_layers.json"
-import { PrepareTheme } from "../Models/ThemeConfig/Conversion/PrepareTheme"
+import {PrepareTheme} from "../Models/ThemeConfig/Conversion/PrepareTheme"
 import * as licenses from "../assets/generated/license_info.json"
 import TagRenderingConfig from "../Models/ThemeConfig/TagRenderingConfig"
-import { FixImages } from "../Models/ThemeConfig/Conversion/FixImages"
+import {FixImages} from "../Models/ThemeConfig/Conversion/FixImages"
 import Svg from "../Svg"
+import {DoesImageExist, PrevalidateTheme, ValidateThemeAndLayers} from "../Models/ThemeConfig/Conversion/Validation";
 
 export default class DetermineLayout {
     private static readonly _knownImages = new Set(Array.from(licenses).map((l) => l.path))
@@ -129,11 +130,11 @@ export default class DetermineLayout {
             }),
             json !== undefined
                 ? new SubtleButton(Svg.download_svg(), "Download the JSON file").onClick(() => {
-                      Utils.offerContentsAsDownloadableFile(
-                          JSON.stringify(json, null, "  "),
-                          "theme_definition.json"
-                      )
-                  })
+                    Utils.offerContentsAsDownloadableFile(
+                        JSON.stringify(json, null, "  "),
+                        "theme_definition.json"
+                    )
+                })
                 : undefined,
         ])
             .SetClass("flex flex-col clickable")
@@ -179,6 +180,23 @@ export default class DetermineLayout {
 
         json.id = forceId ?? json.id
 
+        {
+            let {errors} = new PrevalidateTheme().convert(json, "validation")
+            if (errors.length > 0) {
+                throw "Detected errors: " + errors.join("\n")
+            }
+        }
+        {
+            let {errors} = new ValidateThemeAndLayers(
+                new DoesImageExist(new Set<string>(), _ => true),
+                "",
+                false,
+                SharedTagRenderings.SharedTagRendering
+            ).convert(json, "validation")
+            if (errors.length > 0) {
+                throw "Detected errors: " + errors.join("\n")
+            }
+        }
         return new LayoutConfig(json, false, {
             definitionRaw: JSON.stringify(raw, null, "  "),
             definedAtUrl: sourceUrl,
diff --git a/Models/ThemeConfig/Conversion/Validation.ts b/Models/ThemeConfig/Conversion/Validation.ts
index d7d2acd098..0a996e10ae 100644
--- a/Models/ThemeConfig/Conversion/Validation.ts
+++ b/Models/ThemeConfig/Conversion/Validation.ts
@@ -147,7 +147,7 @@ class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
         const warnings = []
         const information = []
 
-        const theme = new LayoutConfig(json, true)
+        const theme = new LayoutConfig(json, this._isBuiltin)
 
         {
             // Legacy format checks
@@ -168,7 +168,7 @@ class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
                 }
             }
         }
-        {
+        if(this._isBuiltin)  {
             // Check images: are they local, are the licenses there, is the theme icon square, ...
             const images = new ExtractImages(
                 this._isBuiltin,
@@ -224,6 +224,8 @@ class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
         }
 
         try {
+            if(this._isBuiltin){
+
             if (theme.id !== theme.id.toLowerCase()) {
                 errors.push("Theme ids should be in lowercase, but it is " + theme.id)
             }
@@ -250,6 +252,7 @@ class ValidateTheme extends DesugaringStep<LayoutConfigJson> {
                 warnings,
                 information
             )
+            }
             const dups = Utils.Dupiclates(json.layers.map((layer) => layer["id"]))
             if (dups.length > 0) {
                 errors.push(
@@ -298,7 +301,7 @@ export class ValidateThemeAndLayers extends Fuse<LayoutConfigJson> {
         super(
             "Validates a theme and the contained layers",
             new ValidateTheme(doesImageExist, path, isBuiltin, sharedTagRenderings),
-            new On("layers", new Each(new ValidateLayer(undefined, false, doesImageExist)))
+            new On("layers", new Each(new ValidateLayer(undefined, isBuiltin, doesImageExist)))
         )
     }
 }
@@ -506,7 +509,7 @@ export class DetectShadowedMappings extends DesugaringStep<QuestionableTagRender
         {}
     )} is fully matched by a previous mapping (namely ${j}), which matches:
     ${parsedConditions[j].asHumanString(false, false, {})}.
-    
+
     To fix this problem, you can try to:
     - Move the shadowed mapping up
     - Do you want to use a different text in 'question mode'? Add 'hideInAnswer=true' to the first mapping
@@ -700,7 +703,7 @@ export class ValidateLayer extends DesugaringStep<LayerConfigJson> {
         }
 
         try {
-            {
+         if(this._isBuiltin)   {
                 // Some checks for legacy elements
 
                 if (json["overpassTags"] !== undefined) {
@@ -747,7 +750,7 @@ export class ValidateLayer extends DesugaringStep<LayerConfigJson> {
                     warnings.push(context + " has a tagRendering as `isShown`")
                 }
             }
-            {
+           if(this._isBuiltin) {
                 // Check location of layer file
                 const expected: string = `assets/layers/${json.id}/${json.id}.json`
                 if (this._path != undefined && this._path.indexOf(expected) < 0) {
@@ -795,6 +798,7 @@ export class ValidateLayer extends DesugaringStep<LayerConfigJson> {
                     }
                 }
             }
+
             if (json.tagRenderings !== undefined) {
                 const r = new On(
                     "tagRenderings",
diff --git a/scripts/ScriptUtils.ts b/scripts/ScriptUtils.ts
index 21d7079aeb..6dc454c55b 100644
--- a/scripts/ScriptUtils.ts
+++ b/scripts/ScriptUtils.ts
@@ -122,7 +122,7 @@ export default class ScriptUtils {
         return root.svg
     }
 
-    public static async ReadSvgSync(path: string, callback: (svg: any) => void): Promise<any> {
+    public static ReadSvgSync(path: string, callback: (svg: any) => void): any {
         xml2js.parseString(readFileSync(path, "UTF8"), { async: false }, (err, root) => {
             if (err) {
                 throw err