When editing a text box on a smaller breakpoint you will see that there is no option to save the changes of the paragraph style.
IF you want to save the changes - there is another way to modify the style for the entire breakpoint without affecting your Desktop style.
- DO NOT click into the textbox. Make sure you have nothing selected
- Click VIEW > PARAGRAPH STYLES to open the styles window
- Select the style you would like to edit
- Make your modifications in the Design Panel and it will automatically save for this style and breakpoint.