/* Display usernames in bold red */
body a.link-user,
body.theme-light a.link-user,
body.theme-dark a.link-user,
body.theme-black a.link-user {
    color: #ff5555;
    font-weight: bold;
}

/* WARNING: UGLY
 Display username mentions (e.g. @apoctr) in bold red,
Have to match /user/ in href instead of @ in text because of CSS limitations AFAIK.
---
Avoids changing colour of text in PM message of other users (:not([href$="new_message"]),
Avoids changing colour of own username at top of screen - (:not(.logged-in-user-username)).
---
If anyone has a better way to achieve the same thing, let me know.
*/
body a[href^="/user/"]:not([href$="/new_message"]):not(.logged-in-user-username),
body.theme-light a[href^="/user/"]:not([href$="/new_message"]):not(.logged-in-user-username),
body.theme-dark a[href^="/user/"]:not([href$="/new_message"]):not(.logged-in-user-username),
body.theme-black a[href^="/user/"]:not([href$="/new_message"]):not(.logged-in-user-username) {
    color: #ff5555;
    font-weight: bold;
}