[bug] Email template does not support dark theme

[pandafy]

Describe the bug
Some email clients force the dark theme on the email content. Due to this, the logo becomes nearly invisibly in dark theme

Expected outcome
We need to find a way to override the dark theme in this scenario and force light theme.

[aswinms926]

@pandafy Hi! I would like to work on this issue.

[openwisp-companion]

Hi @aswinms926 👋,

Thank you for your interest in contributing to OpenWISP! 🎉

According to our contributing guidelines, you don't need to wait to be assigned to start working on an issue.
We encourage you to:

1. Fork the repository and start working on your solution
2. Open a Pull Request (PR) as soon as possible - even as a draft if it's still in progress
3. Link your PR to this issue by including Fixes #490 in the PR description

Note: We suggest Fixes because this appears to be a bug. You can also use:

• Closes #490 for features/changes
• Fixes #490 for bugs
• Related to #490 for PRs that contribute but don't completely solve the issue

Once you open a PR that references this issue, you will be automatically assigned to it.

This approach helps us:

• See your progress and provide early feedback
• Avoid multiple contributors working on the same issue unknowingly
• Keep the contribution process moving smoothly

We look forward to your contribution! If you have any questions, feel free to ask in the PR or check our documentation.

Happy coding! 🚀

[openwisp-companion]

This issue has been automatically assigned to @aswinms926 who opened PR #614 to address it. 🎯