Here are the custom userstyles I’ve developed for opencraft-related tools - just minor tweaks that I find helpful. They may be of interest to others. I’ve added comments to explain each style. 
Jira
/* highlight user mentions in a colour that contrasts with other links */
a.user-hover {
color: #4ca338;
}
/* highlight my mentions in a different colour (can find my mentions quicker) */
a.user-hover[rel="swalladge"] {
color: #ad652f;
}
/* hide avatars in comments (more compact) */
.aui-avatar.aui-avatar-xsmall {
display: none;
}
.user-hover.user-avatar {
padding: 0 !important;
}
/* Improve readability/discoverability of inline `{{monospace code}}` */
tt {
background: rgba(68, 68, 68, 0.1) none repeat scroll 0% 0%;
}
/* bottom border for the sprint board columns titles is ugly */
.ghx-column-headers .ghx-column {
border-bottom: none;
}
Mattermost
/* links in mattermost are difficult to see (at least with the light theme) */
a.markdown__link {
color: black;
background-color: lightblue;
}
Gitlab
/* Force the "request to merge ..." box into two lines.
* This ensures that the branch name is always fully visible.
* I don't like partially hidden data.
* (see screenshots below)
*/
.git-merge-container {
flex-direction: column !important;
align-items: flex-start !important;
}
.git-merge-container .label-branch a {
max-width: unset !important;
}
.git-merge-container .branch-actions {
align-self: unset !important;
margin-left: 0 !important;
}
The above is better explained with screenshots. Before:
After:
Labxchange devstack
/* the iframe resizer doesn't work on localhost in firefox (some security policy about message passing)
* so I hardcode this to very high so I can see xblock content
*/
iframe#iFrameResizer0,
iframe#iFrameResizer1,
iframe#iFrameResizer2,
iframe#iFrameResizer3,
iframe#iFrameResizer4,
iframe#iFrameResizer5 {
height: 3000px;
}
I manage userstyles with Stylus.
Ticket: FAL-2551