Change HTML syntax to better match Material Theme

[lmlorca]

Im not sure if this is how you reopen a PR... Edit: It seems I had to make a new PR, didn't see any option to reopen the old one. For future reference, I copy again the comparison screenshots.

Darker:

Before:

Now:

Ocean:

Before:

Now:

Lighter:

Before:

Now:

Palenight:

Before:

Now:

[kaicataldo]

Thanks! Sorry again for accidentally closing your PR!

[kaicataldo]

LGTM, thank you!

[lmlorca]

Thank you for the fantastic theme. Best vim theme by far!

[kaicataldo]

Thanks for the kind words!