To change WeGlot language dropdown color, you can use these CSS code to Website Tools > Custom CSS.
#1. Language dropdown color on All Pages
div.language-picker-content { background-color: #f1f !important; }
#2. Desktop Only
@media screen and (min-width:992px) { div.language-picker-content { background-color: #f1f !important; } }
#3. Mobile Only
div#multilingual-language-picker-mobile { background-color: #f1f; }
#4. Before Header Scroll
header#header:not(.shrink) div.language-picker-content { background-color: #f1f !important; }
#5. On Header Scroll
header#header.shrink div.language-picker-content { background-color: #f1f !important; }
#6. One Page
First, you need to find Page ID.
Next, use CSS code like this
#collection-66cf20203be85710e0469d44 div.language-picker-content { background-color: #f1f !important; }