How to fix the sub-item in the menu?

Hello!

I’m using the flat theme and it’s working great ??
I was just wondering what I would need to change in my child theme so that the sub-items in the menu are always displayed.
For the moment the sub-menu are only displayed when the mouse hovers the parent menu item or when clicking on the parent menu item. I don’t find it very user-friendly because the arrow showing that there are sub-menus is super small..

Is there any way to either:
– freeze the sib-items in the menu
– Increase the size of the menu font and arrow?

Thanks a lot for the help.
And sorry for these basics question, I tried to add some stuff in the css of my child theme, it works great for items such as h3 / a / etc. but not for things which are in the left panel (menu, site title and site name).