Global settings
In the Score Settings dialog, on the Project page (Notation Style subpage), you can find the following three options for beam appearance in the Beams category:
Thick Beams.
Activate this if you want beams to be displayed as thick lines.
Show Small Slants as Flat Beams.
If this option is activated, beams that would be only slightly slanted are displayed flat.
Slanted Beams only Slightly Slanted.
Activate this if you only want a slightly slanted beam even though there might be a significant pitch difference between the notes under the beam.
Important
These settings are global for all staves.