org-agenda-fontify-priorities

org-agenda-fontify-priorities is a variable defined in `org-agenda.el'.
Its value is

cookies

Documentation:
Non-nil means highlight low and high priorities in agenda.
When t, the highest priority entries are bold, lowest priority italic.
However, settings in `org-priority-faces' will overrule these faces.
When this variable is the symbol `cookies', only fontify the
cookies, not the entire task.
This may also be an association list of priority faces, whose
keys are the character values of `org-highest-priority',
`org-default-priority', and `org-lowest-priority' (the default values
are ?A, ?B, and ?C, respectively). The face may be a named face, a
color as a string, or a list like `(:background "Red")'.
If it is a color, the variable `org-faces-easy-properties'
determines if it is a foreground or a background color.

You can customize this variable.