TL;DR To set default syntax highlighting in Sublime Text 3, open a file with the extension you would like to set (scss, for example), then click:
View > Syntax > Open all with current extension as > CSS
Set default syntax highlighting in Sublime Text 3
Now any file with an SCSS extension will get CSS syntax highlighting (text coloring).
To change the syntax highlighting for just the current open file hit command + shift + P, type the language name and select from the list.
Set syntax highlighting in Sublime Text 3
If the syntax you need is not in the list (Dust, for example), you can install it. I’ll explain how in another post.