Rounded corners on gallery images

I'm trying to create rounded corners on gallery images but haven't been able to track down the correct code for this. Would love some help please. 

Also, I was able to imput code to add backgrounds to the navigation menu, however this only shows up on the home page. How do I add these same elements to all pages? Thank you!! 

https://seal-point-zww3.squarespace.com/

password: puglife 

This is for the gallery rounded images:

.gallery-masonry-item-wrapper {
    border-radius: 20px;}

For the navigation backgrounds, for the first section you just need to add a background image to the section like this:
https://support.squarespace.com/hc/en-us/articles/4424798177549-Section-background-images

Thanks for rounded corner code @DPruitt!

In regards to the navigation menu, I'm actually referring to when you hover over a menu item and the submenu appears. I am trying to add navigation menu hover effects to other pages.  (Apologies, as I'm still learning lingo for the different website components and might have a difficult time communication my intentions.)  

See examples:  Top image is the home page and bottom image is another page but i can't get hover effects/colors to apply to submenus. 😞

Thanks so much for the help!! 

With nav sub background, you can use this CSS code

/* Sub nav background */
.header-nav-folder-content {
    background-color: #c2cccd !important;
}