feat: support themes #44
Description
Define component styles for different themes.
Get a theme from the configuration or from the environment variable "JLV_THEME".
Support config hierarchy (example: use a theme from the home directory if it is set).
See also:
https://github.com/antonmedv/fx/blob/19520743f7e7b6eadc630cf96d3268c83027e5b9/theme.go#L104
Prepare screenshots in the documentation. Make sure dark/light terminal looks OK.