Theme with variable width inside settings

[Heddson]

In the CSS you can use something like this
#{$header_width} .

Check this topic:

Add settings to your Discourse theme

Discourse has the ability for themes to have “settings” that can be added by theme developers to allow site owners to customize themes through UI without having to change any line of code and worry about losing their changes with future updates for the theme. Themes can also alter certain themeable site settings, for more information on that, see the Themeable site settings topic. Adding settings to your theme Adding settings to your theme is a bit different from adding CSS a…