Adjust font-size of Copilot

[teyang-lau]

Is your feature request related to a problem? Please describe.
I am using the copilot which inherits the font-size of html. Not sure if there is anyway to adjust the font-size and other CSS related properties.

Describe the solution you'd like
We should expose some property in the mountChainlitWidget javascript to allow adjustments of font-size and other properties

Describe alternatives you've considered
I tried to use the custom_css in config.toml to specify a .css file that sets the font-size but it does not work.