The logic is similar to https://discuss.flarum.org/d/2372-replace-star-icon-to-something-else
The only catch is that now we use FontAwesome 5, where icons are split between icon styles that are in different font files.
Because the original icon (pencil) is of the kind "solid", you can use the method from that link without any change to apply another solid icon. Lookup the unicode value at https://fontawesome.com/icons?d=gallery&s=solid&m=free (click on an icon and the value will be listed on the icon page under the title after the style and unicode glyph) and use it like below (here bubble icon):
.IndexPage-newDiscussion .icon::before {
content: '\f075';
}
If you want to use an icon from another style you'll have to change the font as well, for example to use the Android icon:
.IndexPage-newDiscussion .icon::before {
font-family: 'Font Awesome 5 Brands';
content: '\f17b';
}