A theme also provides an amazing option for styling your default WordPress login screen you can find the option inside:
Dashboard >> Appearance >> Customize >> WordPress Login Screen.
Custom Logo: Select custom logo to replace default WordPress logo.
Background Image: Select image to set as background image.
Login Screen Background Color: Select colour to set as a background colour.
Login Form Background Color: Select colour to set as a background colour for the login form.
Login Form Text Color: Select colour to set for text in Login Form.
Login Form Button Color: Select colour to set for button colour in Login Form.
Custom CSS: In case you need any custom CSS to apply on Login Screen, write it here.