docs(theming): point chrome:'none' styling at the custom SDT variables (SD-3322)

The content-controls theming table themes the built-in chrome. Add a one-line
note that under chrome:'none' you style controls with the
--sd-content-controls-custom-* variables instead, linking the custom UI guide.

Author: caio-pizzol

apps/docs/editor/theming/custom-themes.mdx

@@ -318,6 +318,8 @@ If you also want tracked-change text inside comment threads to match, set `--sd-
 
 DOCX content controls (SDTs): form fields, dropdowns, date pickers.
 
+These theme the **built-in** chrome (`modules.contentControls.chrome: 'default'`). If you turn the chrome off (`chrome: 'none'`) to draw your own field/clause look, style the controls with the `--sd-content-controls-custom-*` variables instead. See [Custom UI > Content controls](/editor/custom-ui/content-controls).
+
 | Variable | Default | Controls |
 |----------|---------|----------|
 | `--sd-content-controls-block-border` | `#629be7` | Block control border |