We do not generally provide CSS to change breakpoints because these ship with the plugin.
However, you can control the default view/layout, and it is not necessary to use CSS to achieve this with V2.
There are 2 elements involved
The layout selector is shown on the page with gd_loop_actions
The listings are shown with gd_loop
The layout is predetermined by the options applied to gd_loop
If you apply the layout to gd_loop, then remove gd_loop_actions, the layout you choose will be set and the user will not be able to change it.
To make the adjustments, open your search page template and adjust the elements
https://wpgeodirectory.com/docs-v2/geodirectory/personalize/
https://wpgeodirectory.com/docs-v2/geodirectory/page-design/
Let us know how it goes