A theme component can also provide its own Configuration File, e.g. config.toml. There are some restrictions to what can be configured in a theme component, and it is not possible to overwrite settings in the project.
The following settings can be set:
params (global and per language)
menu (global and per language)
outputformats and mediatypes
Theme Description File
In addition to the configuration file, a theme can also provide a theme.toml file that describes the theme, the author and origin etc. See Add Your Hugo Theme to the Showcase.