How to auto-set a Default Boarder

[Tess Baird]

Hello, I'd like to mention that I am new here. 

I am trying to learn how to set a default boarder for all pasted photos on a page. I am moving our entire 500 + word document training manual over to confluence and I am pasting loads of photos. 

It is very time consuming to have to click Boarder Options twice (once to choose the color - we chose Black, and another time to choose the size thickness - we chose the thickest one)

I saw something about CSS stylesheets but I can't seem to find where that is. 

For more context, I am a space admin but work closely with the org admin if I need help from them too. 

Any help or suggestions would be greatly appreciated!