Add custom global variables. Don't forget to restart hugo if you make changes to the .toml files. If you don't these variables you added will not show up.
[params]
myvariable = "testing 123"
[params.general]
mysubvariable = "more tests"
in your theme file.
{{ .Site.Params.myvariable }}
{{ .Site.Params.general.mysubvariable }}