There's no dedicated Style Property for text selection color in XenForo — you'll need to add it manually via CSS.
Go to Admin Panel > Appearance > Styles > [Your Style] > Templates > extra.less and add:
::selection {
background-color: #3b82f6;
color: #fff;
}
::-moz-selection {
background-color: #3b82f6;
color: #fff;
}
Just swap #3b82f6 with whatever color you want. The ::-moz-selection part is for older Firefox versions — doesn't hurt to include it.