mdgreenopu

Hey @Bexxx - welcome to Squarespace! 👋 
A lot of the coding you used in Showit will work for common HTML things, like the a that you'll use to target an active link. But you've already discovered that Squarespace has its own unique selectors for some important things. 😉
The active link in your website header has a background image you can hide with a version @tuanphan 's code. This code below will hide the underline from pages you are actively on, and leave a border hover effect that will be the same color as your nav color, even if you change it in the site style menu: 
.header-nav-item--active a{background:transparent!important} .header-nav-item a:hover{border-bottom:1px solid var(--solidHeaderNavigationColor)!important} If there are other things you want to change, you'll find a ton of free codes on my tutorial blog at insidethesquare.co/squarespace-tutorials
 
I hope all that info helps, and best of luck with your Squarespace project!

I also forgot about your second question - borders! Those can be tricky for images. If you have one set to "fill" instead of fit, or a shape, you can use a basic border code. All content blocks get a unique block id that you can find with a free Chrome extension that a lot of us use ( https://insidethesquare.co/chromeext )
Here is a tutorial about creating code for specific blocks: https://insidethesquare.co/squarespace-tutorials/css-targeting-tricks
and here is an article about borders: https://insidethesquare.co/borders

This is the custom css I had been using, with details edited to fit my site (sizing, etc).  I re-entered it fresh, added my image url, and the dots remained.  
I started playing around with sizing and for some reason setting the font-size at 2em and margin-top at .01em made the dots disappear.  No clue why but I am taking this as a win!  I hope this helps someone else.
 
/*Add logo to display in mobile*/ #mobile-navigation-title a {   color: transparent;   background-image: url("YOUR_IMAGE_URL_HERE");   background-size: contain;   background-position: center;   background-repeat: no-repeat;   font-size: 2.5em; } /*Adjust mobile hamburgernav position*/ #mobile-navigation-label {   margin-top: 1em; }