feat: rework login/signUpIn classes and css vars

[cmunns]

apply css variables and general base theme to login component.

[codesandbox-ci]

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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