fix themes v3 not loading

changelog.d/themes3.fix

@@ -0,0 +1 @@
+Fixed themes 3 not loading in appearance tab

src/api/helpers.js

@@ -71,6 +71,7 @@ export const promisedRequest = async ({
   url,
   payload,
   formData,
+  forceContentType,
   cache,
   credentials,
   headers = {},
@@ -79,7 +80,7 @@ export const promisedRequest = async ({
     method,
     credentials: 'same-origin',
     headers: {
-      Accept: 'application/json',
+      Accept: forceContentType ?? 'application/json',
       ...headers,
     },
   }
@@ -114,7 +115,7 @@ export const promisedRequest = async ({
     )
     if (contentLength === 0) return null
 
-    switch (contentType) {
+    switch (forceContentType ?? contentType) {
       case 'text/plain':
         return await response.text()
       case 'application/json':

src/services/style_setter/style_setter.js

@@ -307,7 +307,9 @@ export const applyStyleConfig = (input) => {
   adoptStyleSheets()
 }
 
-export const getResourcesIndex = async (url, parser = (x) => x) => {
+const noop = (x) => x
+
+export const getResourcesIndex = async (url, parser = noop) => {
   const cache = 'no-store'
   const customUrl = url.replace(/\.(\w+)$/, '.custom.$1')
   let builtin
@@ -324,6 +326,7 @@ export const getResourcesIndex = async (url, parser = (x) => x) => {
             promisedRequest({
               url: v,
               cache,
+              forceContentType: parser === noop ? null : 'text/plain',
             })
               .then(({ data: text }) => parser(text))
               .catch((e) => {