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
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.
If the syntax you need is not in the list (Dust, for example), you can install it. I’ll explain how in another post.