feat: rework login/signUpIn classes and css vars

[cmunns]

apply css variables and general base theme to login component.

[codesandbox-ci[bot]]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit a6bd64545b9eee004e7bdb84d6f83f1567a8cbab:

Sandbox	Source
@w3ui/example-react-file-upload	Configuration
@w3ui/example-react-sign-up-in	Configuration
@w3ui/example-react-uploads-list	Configuration
@w3ui/example-solid-file-upload	Configuration
@w3ui/example-solid-sign-up-in	Configuration
@w3ui/example-solid-uploads-list	Configuration
@w3ui/example-vue-file-upload	Configuration
@w3ui/example-vue-sign-up-in	Configuration
@w3ui/example-vue-uploads-list	Configuration

[jchris]

apologies for my terrible sketch, but does it make sense to move the w3ui logo "inside" the widget? so that people know what they are registering for.

I guess it could be part of the button background image, like this