Correct Code

.fa, .fas, .sys-icon, .far, .sys-icon.far, .fal, .sys-icon.fal, .fad, .sys-icon.fad {
font-family: 'sys-user-fal';
font-size: large;
}

(font-weight and font-style not really needed in Custom Styles)

Depending on what Style you're using.

Example...

Code above will mess with your 'solid' and change to 'light'. So, in order to fix that...

Regular

font-family: 'sys-user-far'; 

Light

font-family: 'sys-user-fal';

Duotone

font-family: 'sys-user-fad';

Solid (font-family not needed)

.fa, .fas, .sys-icon, .far, .sys-icon.far, .fal, .sys-icon.fal, .fad, .sys-icon.fad {
font-size: large;
}