refactor(asset-metadata): remove unused theme setting and CSS variable overrides

- Remove `theme` setting from `screenly.yml`, `screenly_qc.yml`, and README
- Remove unused `setProperty` calls for `--app-bg`, `--card-bg`, `--card-border`,
  `--text-primary`, `--text-secondary`, `--chip-bg`, and `--chip-color`
- Replace with a single `--theme-color-primary` set from branding colors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nicomiguelino

edge-apps/asset-metadata/README.md

@@ -55,7 +55,6 @@ Screenshots are saved to the `screenshots/` directory.
 
 ## Configuration
 
-| Setting          | Description                               | Type     | Default |
-| ---------------- | ----------------------------------------- | -------- | ------- |
-| `display_errors` | Display errors on screen for debugging    | optional | `false` |
-| `theme`          | Visual theme for the app (`light`/`dark`) | required | `light` |
+| Setting          | Description                            | Type     | Default |
+| ---------------- | -------------------------------------- | -------- | ------- |
+| `display_errors` | Display errors on screen for debugging | optional | `false` |

edge-apps/asset-metadata/screenly.yml

@@ -42,10 +42,3 @@ settings:
     help_text: |
       Enter your Google Tag Manager container ID to enable tracking and analytics.
     is_global: true
-  theme:
-    type: string
-    default_value: light
-    title: Theme
-    optional: false
-    help_text: |
-      Select the visual theme for the app ('light' or 'dark').

edge-apps/asset-metadata/screenly_qc.yml

@@ -42,10 +42,3 @@ settings:
     help_text: |
       Enter your Google Tag Manager container ID to enable tracking and analytics.
     is_global: true
-  theme:
-    type: string
-    default_value: light
-    title: Theme
-    optional: false
-    help_text: |
-      Select the visual theme for the app ('light' or 'dark').