I love IRC and this is the best macOS client for it.
I created my own Monokai Night theme for it that I love.
I can inspect HTML of the window by going to
HELP -> Advanced -> Enter Developer Mode and then right clicking on what I want to inspect.
Right click on chat background and
Force Reload Style will reapply the styles you made in the active theme (as well as the JS code).
CMD + enter when typing will render text as
/me command. OPT + CMD will allow to write multi line text.