Hi @tindeks2,
Thanks for the question. There is currently no way to do this via the customizer. I’ll look into including this feature in a future update.
In the meantime, you could accomplish this by adding the following to the bottom of the stylesheet:
#secondary {
max-width: 20% !important;
}
#primary {
max-width: 75% !important;
}
Replacing the 20% and 75% with your preferred widths for the sidebar and content respectively.