Added theme color meta tag

[FlopCoder35]

Changes Made

• Added a <meta name="theme-color" content="#3c790a"> tag to src/index.html.

Reason

This change enhances the user experience on mobile devices by setting a consistent theme color for the browser's address bar.

No related issues were addressed.

[coliff]

Thanks for the PR... If we were to add this we'd need the documentation updating and a build made... (change should be in dist/ too)

[adamaveray]

@coliff The index.html file already has a theme-color meta tag (added in this commit from 2018 which coincidentally it looks like you made!), so this PR would add a second entry. I think it can safely be closed.