[desktop] Make carousel arrow buttons more prominent

[casillasenrique]

Describe the feature

On desktop the arrow buttons for toggling between migrants in the migrant select page are quite small, and may be difficult to notice. May benefit from being larger, or have some clearer indication that you can click on them.

Priority

0.5 (Nitpick)

Additional Notes and Information

During the study, some users didn't realize you could select a different migrant.