I made a couple of nice themes for boards.ie. Check them out!

'no styles installed for this site'

is the message i get when i click on the icon😐️

assuiming you have stylus installed.

open boards site.

click stylus

hover over write style for and click when www.boards.ie is black and this url is grey

when text box appears, copy the code into it, click save and go back to boards.

this is example code which works for me as of now.

themeHeader {
    display: none !important;
}
.Frame-content, .Breadcrumbs {
    margin-top: 0 !important;
    margin-bottom: 0 !important;
}
[class^="vanilla-"][class$="-container-fullGutter-container-fullGutter-container-fullGutter"], .Breadcrumbs {
    max-width: unset !important;
    padding-left: 15px !important;
    padding-right: 15px !important;
}
.Container {
    max-width: none !important;
    padding-left: 15px !important;
    padding-right: 15px !important;
}

/* threads */
.postbit-message, .Message.userContent {
    padding: 0px !important;
    margin: 0px !important;
}
.postbit-message {
    padding-left: 10px !important;
    padding-right: 10px !important;
}
.postbit-message ~ br {
    display: none !important;
}
.postbit-userinfo br {
    display: none !important;
}
.postbit-userinfo {
    padding-top: 8px !important;
    height: 100% !important;
}
.ItemComment::after, .ItemComment::before, .MessageList::before, .MessageList::after {
    display: none !important;
}
.MessageList {
    margin-bottom: 0 !important;
}
.embedExternal {
    margin: 0 !important;
    width: 100% !important;
    clear: none !important;
}
.vanilla-d7cut4-collapsableContent-heightContainer {
    height: auto !important;
}
.vanilla-toe1gy-collapsableContent-gradient {
    opacity: 0 !important;
}
.vanilla-1vww8zj-collapsableContent-footer {
    display: none !important;
}
.Reactions {
    padding: 0 !important;
    margin: 0 !important;
    margin-right: 10px !important;
    position: unset !important;
    float: right !important;
}
.postbit-postbody .clearleft {
    clear: both !important;
}
.postbit-header time {
    visibility: hidden !important;
}
.postbit-header time::after {
    content: attr(title) !important;
    visibility: visible !important;
    position: absolute !important;
    left: 25px !important;
}
.bodybox-wrap {
    padding-left: 100px !important;
}
#latest + .Comment .postbit-header {
    font-weight: bold !important;
}

/* forums */
.forum-threadlist-thread-lastpost {
    float: left !important;
}
.forum-threadlist-thread-lastposter {
    float:right !important;
    width: unset !important;
}
.Category {
    float: right !important;
}
.Frame-content .HomepageTitle {
    display: block !important;
}
/*Remove Twitter/Facebook buttons */
.ReactSprite {
    display: none;
}

as has been said by others, as they fix the site, these will break and need modded

I have turned this into a browser extension (and it now includes a dark mode setting to add to the features above). See details here: https://www.boards.ie/discussion/2058208910/boardsie-enhancement-suite-browser-extension/p1

I'm likely after doing something wrong. Doesn't look great for me.