[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?