feat: add css parameter for custom CSS injection in screenshots

sdk/python/README.md

@@ -107,17 +107,21 @@ single_hide = snap.capture(
 )
 ```
 
-### Combined Dark Mode + Element Hiding
+### Custom CSS Injection
 
 ```python
-# Perfect for clean marketing screenshots
-marketing_shot = snap.capture(
-    "https://your-saas-app.com",
+# Inject custom CSS before capture
+styled = snap.capture(
+    "https://example.com",
+    css='body { background: #1a1a2e !important; color: #eee !important }',
+)
+
+# Combine with other options
+combined = snap.capture(
+    "https://example.com",
+    css=".hero { padding: 80px 0 } h1 { font-size: 48px }",
     dark_mode=True,
-    hide_selectors=[".dev-banner", ".beta-notice"],
-    width=1920,
-    height=1080,
-    device_scale=2,
+    hide_selectors=[".cookie-banner"],
 )
 ```
 
@@ -154,6 +158,7 @@ except SnapAPIError as e:
 | `wait_until` | `str` | `"domcontentloaded"` | Load event |
 | `dark_mode` | `bool` | `False` | Emulate prefers-color-scheme: dark |
 | `hide_selectors` | `list` | — | CSS selectors to hide before capture |
+| `css` | `str` | — | Custom CSS to inject before capture (max 5000 chars) |
 
 ### `snap.health() -> dict`