You can't set the post bar font, which belongs to the sign-in permission. For the same post bar, you can use the bold function for continuous login 10 days, and the red function for continuous login for 20 days.

Font bold

When you speak in this post bar, you have to click the bold button in the upper part of the reply box to make it bold.

Add red to the font

When you speak in this post bar, click a floating red button in the upper part of the reply box to add red. Moreover, one post bar can only be bold red after signing in for ten days, and the signing days of other bars are accumulated separately.