You can use html tags to customize your notification's text, using for example <span> elements:

To achieve this, change the "Message" of your notification to:

Markdown is a simple way to format text that looks great on any device. It doesn’t do anything fancy like change the font size, color, or type — just the essentials, using keyboard symbols you already know.

This option is supported in all notifications EXCEPT for COMBO and REVIEWS:
​Stream, Informational and Counter

Basic styles:

To add bold text to your notification, simply wrap your words with 2 asterisks (*).
​Example: 

Result:

To add italic text to your notification, simply wrap your words with single asterisk (*).
​Example:

Result:

To make your text bold + italic, simply wrap your words with 3 asterisks (*).
​Example: 

Result:

To add a link in your notification text, put the text link between square brackets followed by the link itself in round brackets, e.g. [text] followed by (link)
​Example: 

Result:

To add code to your notification, wrap the text in back-tick symbol `
e.g. `text`
​Example: 

Result:

To learn more about available markdown formatting/styles, visit https://commonmark.org/help/