fix: light and dark mode handling in Embed Sidebar mode (#115)

#114 

Before:
<img height="600" alt="image"
src="https://github.com/user-attachments/assets/32803034-163d-42b3-b6df-de76ad257cf8"
/>

After:
<img height="600" alt="image"
src="https://github.com/user-attachments/assets/cca25c62-db54-4b3a-ab63-43d3f9dfca71"
/>

Author: wjiayis

webapp/_webapp/src/index.css

@@ -219,7 +219,12 @@ object {
   border: 1px solid;
   background-color: transparent;
   overflow: hidden;
-  @apply overflow-hidden w-full h-full dark:!border-default-200 !border-default-200;
+  @apply overflow-hidden w-full h-full dark:!border-default-200 !border-default-200 text-default-800;
+
+  textarea,
+  input {
+    background-color: var(--pd-default-bg);
+  }
 }
 
 .pd-app-control-title-bar {