In the new client you can simply send your code in markdown formatting and it will be rendered with a monospace font.
Put your code in between those backtick characters to send code blocks.
(This example contains zero width whitespaces to preserve formatting)
For more possibilities look at the markdown standard (Markdown Cheatsheet · adam-p/markdown-here Wiki (github.com)).
If you really want to change the font of the entire client you need to create your own custom theme. This is not an easy task, but you can look at my theme (github.com) for inspiration.