[Feature Request] add option to set background colors for toast

[daBlesr]

Hi, thank you for providing this crate.
I find the toasts difficult to see/notice as there is little contrast with the background color of the app.
Possible ways to make the toasters more noticeable:

• I'd like to be able to configure that e.g. for success toasters the entire background of the toaster becomes green.
• Configure shadow or border.

Is this something you agree with?

[woelper]

Very good idea. A shadow would work well I suppose! I also just ran into an issue where the toast was barely visible.

[woelper]

I opened a PR with optional shadow support:
#33