Right-clicking on the Cell styles icon has no effect. Left-clicking on this icon only opens the window with the cell styles below it.
Unfortunately, you do not describe what you actually want to achieve.
My guess:
You want to open LibreOffice Calc and have the blank sheet use the font and size you want in all cells.
My tip for this:
Right-click on the cell style “Default” and choose New
.
In the “Cell style” dialog, enter an incoming name, e.g. “My Default Text”.
Select the Font
tab and set the desired font, font style and size and click OK
.
Now select the entire table (klick top left corner - header rows).
Now double-click on your new cell style.
Now all cells should be able to execute your font preferences.
If you have multiple tables open repeat this there as well.
Finally, save your document as a document template, File>Document Templates>Save as Document Template
.
In the “Save as Document Template” dialog, enter a name for the template and choose a template category.
And put a check mark at “Set document template as default”.
Now Calc should include your font defaults when you open it again.