[Feature Request] Custom background transparency

[YunYouJun]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
I want a translucent effect like iTerm.
How about adding a configuration to custom opacity?

Additional context
Add any other context or screenshots about the feature request here.

[YunYouJun]

Maybe we just need add opacity property for here?

.terms_terms {
  background: url("file://${imagePath}") center;
  background-size: cover;
  opacity: ${opacity}
}

https://github.com/klaussinani/hyper-pokemon/blob/98abad0879922743d07c0f2c6f97d9c0aa04f5c3/index.js#L128