docs: Superset 6.1 documentation catch-up — batch 4 (#39446)

Co-authored-by: Superset Dev <dev@superset.apache.org>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Michael S. Molina <70410625+michael-s-molina@users.noreply.github.com>

docs/admin_docs/configuration/cache.mdx

@@ -178,6 +178,20 @@ Then on configuration:
 WEBDRIVER_AUTH_FUNC = auth_driver
 ```
 
+## ETag Support for Thumbnails
+
+Thumbnail and screenshot endpoints return `ETag` response headers based on the cached content digest. Clients can use conditional requests to avoid downloading unchanged images:
+
+```
+GET /api/v1/chart/42/thumbnail/
+If-None-Match: "abc123..."
+
+→ 304 Not Modified  (if unchanged)
+→ 200 OK            (with new image if changed)
+```
+
+This is particularly useful for embedded dashboards and external integrations that periodically poll for updated screenshots — unchanged thumbnails return immediately with no payload.
+
 ## Distributed Coordination Backend
 
 Superset supports an optional distributed coordination (`DISTRIBUTED_COORDINATION_CONFIG`) for

docs/admin_docs/configuration/configuring-superset.mdx

@@ -372,6 +372,26 @@ CUSTOM_SECURITY_MANAGER = CustomSsoSecurityManager
   ]
   ```
 
+### PKCE Support
+
+For public OAuth2 clients that cannot securely store a client secret, enable Proof Key for Code Exchange (PKCE) by adding `code_challenge_method` to the `remote_app` configuration:
+
+```python
+OAUTH_PROVIDERS = [
+    {
+        'name': 'myProvider',
+        'remote_app': {
+            'client_id': 'myClientId',
+            'client_secret': 'mySecret',  # may be empty for pure public clients
+            'code_challenge_method': 'S256',  # enables PKCE
+            'server_metadata_url': 'https://myAuthorizationServer/.well-known/openid-configuration'
+        }
+    }
+]
+```
+
+PKCE (`S256`) is recommended for all OAuth2 flows, even when a client secret is present, as it protects against authorization code interception attacks.
+
 ## LDAP Authentication
 
 FAB supports authenticating user credentials against an LDAP server.

docs/admin_docs/configuration/theming.mdx

@@ -341,11 +341,25 @@ Available chart types for `echartsOptionsOverridesByChartType`:
 - `echarts_heatmap` - Heatmaps
 - `echarts_mixed_timeseries` - Mixed time series
 
+### Array Property Overrides
+
+Array properties (such as color palettes) are fully supported in overrides. Arrays are **replaced entirely** rather than merged, so specify the complete array:
+
+```python
+THEME_DEFAULT = {
+    "token": { ... },
+    "echartsOptionsOverrides": {
+        # Replace the default color palette for all ECharts visualizations
+        "color": ["#1f77b4", "#ff7f0e", "#2ca02c", "#d62728", "#9467bd", "#8c564b"]
+    }
+}
+```
+
 ### Best Practices
 
 1. **Start with global overrides** for consistent styling across all charts
 2. **Use chart-specific overrides** for unique requirements per visualization type
-3. **Test thoroughly** as overrides use deep merge - nested objects are combined, but arrays are completely replaced
+3. **Test thoroughly** as overrides use deep merge for objects, but arrays are completely replaced — always specify the full array value
 4. **Document your overrides** to help team members understand custom styling
 5. **Consider performance** - complex overrides may impact chart rendering speed