[FIX] spreadsheet: Fix dark mode assets

The dark mode assets were mistakenly loaded in the default backend
assets, effectively overriding the light theme for spreadsheet 100% of
the time.

closes odoo/odoo#129339

X-original-commit: 97cf8bf7f757b82bdecd6a4531d6b00fcd591bd9
Signed-off-by: Lucas Lefèvre (lul) <lul@odoo.com>
Signed-off-by: Rémi Rahir (rar) <rar@odoo.com>

addons/spreadsheet/__manifest__.py

@@ -68,6 +68,10 @@
             'spreadsheet/static/src/**/*.scss',
             'spreadsheet/static/src/assets_backend/**/*',
             ('remove', 'spreadsheet/static/src/public_readonly_app/**/*.scss'),
+            ('remove', 'spreadsheet/static/src/**/*.dark.scss'),
+        ],
+        "web.dark_mode_assets_backend": [
+            'spreadsheet/static/src/**/*.dark.scss',
         ],
         'web.qunit_suite_tests': [
             'spreadsheet/static/tests/**/*',