If you fix the value of 'Width For Main Area' then you need to set corresponding values for 'Width of sidebar(s) in pixels' and 'Width of web pages in pixels', and vice versa. See the section "The 'Themes' Window" in the main help for some example values.
If your aim is to disable the responsive behaviour, this can be done simply by setting 'Adapt Page Layouts To Screen Size' to 'False', under ‘Appearance Settings’ in the ‘General’ tab of the Site Options. This setting is also documented in the same section of the help.
If your aim is to disable the responsive behaviour, this can be done simply by setting 'Adapt Page Layouts To Screen Size' to 'False', under ‘Appearance Settings’ in the ‘General’ tab of the Site Options. This setting is also documented in the same section of the help.
Comment