Some basic settings for a site are located in the Site.variables.less file. The Site.variables.less file is used to set variables which are used in styling a site and will impact most presentation pages.
The Site.variables.less file is located in AppData/Site/
Typically change colour of toolbar, buttons, icons and text in the less file.
Strategy when migrating from version 4 or when creating a new site is
Start with a new css, copy a template. Also use a profile.config based on template.
Make changes to the Site.variables.less file to get the basic colours and styles.
Then move on and make changes in the css file for the site.
Note that for changes in the less file to show on the site, the site has to be resetted.
It is important all variables are declared and exist in the file.