How does one create and implement a new theme?

Am having difficulty creating a new theme in SuiteCRM 7.8.8. In previous versions, I could simply copy a whole “theme” directory to, say “custom/themes/MyTheme”, make required tweaks to the .css files (mainly style.css), and choose the new theme in my profile.

I have tried this in the current version, and it breaks a number of layouts - attached is one particular “before” and “after” example (n.b. this is immediately after performing the copy - the only file that has been changed is themedef.php).

If the method of creating a custom theme has changed, could someone point me in the direction of the relevant documentation, please? I assume that there is also some procedure for creating the css files, by “compiling” the “scss” files into css files. This is something new to me. Does SuiteCRM contain a command line tool for performing this compilation?

Sorry - had trouble with the attachments; here, hopefully, is the “after” version :