Menu CSS for Mobile

I created some custom CSS in a CSS editor for the off-canvas menu. It works great but on the iPhone mobile format, the menu opens up (expands) and briefly shows the blue background color before it disappears into all white. The menu is remaining open as it should, it just looks like it's not showing the blue background color when the menu is open on mobile devices.
Can you help and give any recommendation as to what CSS I can add or how I can adjust what I have?
Thank you!
Here is what I have so far:
#aside-menu {
background-color: #3981B4;
}

#aside-menu li {
font-size: 1.5rem;
}

#aside-menu li a,
#aside-menu li li a {
color: #54427E;
}

#aside-menu li a:hover,
#aside-menu li li a:hover,
#aside-menu li.current_page_item > a {
color: #3AAD79;
}

#close-menu {
color: #54427E;
#gk-menu-top, #gk-menu-bottom {
color: #54427E;
}

#aside-navigation > div > ul {
padding-bottom: 0;
}

Hi,
Could you please provide me with a URL to your website, either here or via PM (click the “Private Message” text underneath my avatar) so that I may analyze it?