I need to place custom css that has a bad looking, old fashioned scroll bar instead of a thin and modern one
By default, not exist "fine and modern" scroll bar. This can be configured by your hand only, via CSS or JavaScript.
Here is css snippet for scroll bar, that you can set as you like. Replace "your-scroll-block-class" to your own.
It is so much code, because for different browsers it is configured in different ways.
.your-scroll-block-class {
scrollbar-color: #8643b7 #eee;
scrollbar-width: thin;
}
.your-scroll-block-class::-webkit-scrollbar {
background-color: #eee;
}
.your-scroll-block-class::-webkit-scrollbar-thumb {
background-color: #8643b7;
}
.your-scroll-block-class::-webkit-scrollbar-thumb:hover {
background-color: #ad61e4;
}
.your-scroll-block-class::-webkit-scrollbar {
width: 8px;
}
.your-scroll-block-class::-webkit-scrollbar-button:vertical:start:decrement {
background: linear-gradient(120deg, rgba(0, 0, 0, 0) 40%, rgba(0, 0, 0, 0) 41%),
linear-gradient(240deg, rgba(0, 0, 0, 0) 40%, rgba(0, 0, 0, 0) 41%),
linear-gradient(0deg, rgba(0, 0, 0, 0) 30%, rgba(0, 0, 0, 0) 31%);
background-color: #ffffff;
}
.your-scroll-block-class::-webkit-scrollbar-button:vertical:end:increment {
background:
linear-gradient(300deg, rgba(0, 0, 0, 0) 40%, rgba(0, 0, 0, 0) 41%),
linear-gradient(60deg, rgba(0, 0, 0, 0) 40%, rgba(0, 0, 0, 0) 41%),
linear-gradient(180deg, rgba(0, 0, 0, 0) 30%, rgba(0, 0, 0, 0) 31%);
background-color: #ffffff;
}
.your-scroll-block-class::-webkit-scrollbar-button:horizontal:start:decrement {
background:
linear-gradient(30deg, rgba(0, 0, 0, 0) 40%, rgba(0, 0, 0, 0) 41%),
linear-gradient(150deg, rgba(0, 0, 0, 0) 40%, rgba(0, 0, 0, 0) 41%),
linear-gradient(270deg, rgba(0, 0, 0, 0) 30%, rgba(0, 0, 0, 0) 31%);
background-color: #ffffff;
}
.your-scroll-block-class::-webkit-scrollbar-button:horizontal:end:increment {
background:
linear-gradient(210deg, rgba(0, 0, 0, 0) 40%, rgba(0, 0, 0, 0) 41%),
linear-gradient(330deg, rgba(0, 0, 0, 0) 40%, rgba(0, 0, 0, 0) 41%),
linear-gradient(90deg, rgba(0, 0, 0, 0) 30%, rgba(0, 0, 0, 0) 31%);
background-color: #ffffff;
}
It makes such scroll bar

It also applies scroll bar to the horizontal axis which is 100% unnecessary.
It also applies the scroll bar (with no actual effect, the scroll bar has no interaction) even when the height of the submenu is less than the max-height! Insane!
Try this code instead of previus
.sp-dropdown-sub .sp-dropdown-items{
max-height: 300px;
overflow-y: auto;
}
Looking at your menu as a whole, I think you must redistribute it on another menus, and not do everything in one menu. Check out examples of well-known online stores. You will not see there such nesting as you have. Because it contradicts usability, user convenience and screens size. Typically, different sections are redistributed to different menus.
You need to increase your UX design skills. Or see some lessons about UX design.
Classic scheme for online store and any catalog

At 1:28 you can see me applying AND previewing multi column view. After I save successfully, you will se at 2:31 that NO change takes place. What is wrong?
Sorry. I'm not support team member. To find a reason need access to your site