Soulver draws inspiration from programming language editors by automatically highlighting various parts of your expression (numbers, variables, operators, etc) in different colors.
Like programming languages & markdown, sheets in Soulver are saved as plain text.
This means you cannot manually adjust the styling attributes of specific selections of text like you would in Pages or Microsoft Word.
Customizing highlighting colors
Customize the colors Soulver used to indicate the various parts of your expression in the Styling settings pane:
Soulver's styling settings in the Mac version
Monospaced digits
From the Font popup list and select System Default (monospaced digits)