Remove notes and add a plain CSS example

Author: mme

docs/TODO-POPUP.md

@@ -37,6 +37,14 @@ For example, to set the primary color to purple:
 To further customize the chat window, you can override the CSS classes defined
 [here](https://github.com/CopilotKit/CopilotKit/blob/main/CopilotKit/packages/react-ui/src/css/).
 
+For example:
+
+```css
+.copilotKitButton {
+  border-radius: 0;
+}
+```
+
 ## Props
 
 <ResponseField name="instructions" type="string">

docs/reference/CopilotKitPopup.mdx

@@ -45,6 +45,14 @@ For example, to set the primary color to purple:
 To further customize the chat window, you can override the CSS classes defined
 [here](https://github.com/CopilotKit/CopilotKit/blob/main/CopilotKit/packages/react-ui/src/css/).
 
+For example:
+
+```css
+.copilotKitButton {
+  border-radius: 0;
+}
+```
+
 ## Props
 
 <ResponseField name="instructions" type="string">