There's already a style set for an icon to display there actually.
But since the 'Material Icons Font' is no longer used anymore, and SVG icons are used instead, that icon appears as a replacement glyph (rectangle symbol) because the resource (font link) is no longer available.
You can either add the font link to the head of your site (Admin > Layout > Head links) :
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Material+Icons+Outlined&display=swap"/>
Or if you don't want to load the whole font to prevent slowing down your site, you can replace it with an HTML symbol instead, by adding this to (Admin > Theme Options > Custom CSS):
.qa-q-item-title.qa-q-read:before {
content: "\2611";
}