What you need are two things:
- A syntax definition for the particular language. In the videa GLSL is used, which is available via Package Control.
- A color scheme, which sets the
italic flag for certain scopes.
The syntax definition is used to tokenize source code and assign a scope to each token. This scope is assigned a color or character style by a color scheme.
The video seems to use Monokai, the former default color scheme of Sublime Text.
The easiest way to edit existing color schemes might be PackageDev. Once installed, you can simply run “Edit Current Color Scheme” to manipulate it just like settings.
The following example shows a simple rule, which marks keywords to be rendered italic.