[TINY] Some colors are hard coded instead of using grist-var-colors.

[hexaltation]

Describe the current behavior

Start a new document background on the new page is hard coded to #DCF4EB . For custom.css to properly apply it would be better to set it to --grist-color-selection-opaque.

Steps to reproduce

1. overload --grist-color-selection-opaque in custom.css with some distinctive color.
2. see that it don't apply for Start a new document background

Describe the expected behavior

The base color overloaded in custom.css must apply to every element in grist.

Where have you encountered this bug?

• [ ] On docs.getgrist.com
• [X] On a self-hosted instance

Instance information (when self-hosting only)

• Grist instance:

  • Version: 1.2.0
  • URL (if it's OK for you to share it): grist.numerique.gouv.fr
  • Installation mode: kubernetes
  • Architecture: multi-workers

• Browser name, version and platforms on which you could reproduce the bug: firefox 130.0.1