[custom_template_shogun160_battery_info] fix visablity when force_bac…

[shogun160]

…kground_color = true

Fill with var(--card-background-color) instead off none to make battery state readable when force_background_color = true

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• I have followed the contribution guidelines
  • This PR is for a custom-card or documentation change and therefore directed to the main branch.
  • This PR is for a official card or any other directly to the integration related change and therefore directed to the release branch.