Add dark mode logo option

[declan-haojin]

Sometimes the website may have different logos for light and dark mode. It would be great to have the option to specify a dark mode logo in the params.toml file. If no dark mode logo is specified, the default logo is used for both modes. If a dark mode logo is specified, the default logo is used for light mode and the dark mode logo is used for dark mode.

[jpanther]

This is now included in the next release.