ms-teams.css

/* tweak Microsoft Teams Web client style */
/* weaming */
.teams-title,
.icons-appbar-download, .app-bar-text,
#app-bar-help-button, #discover-apps-button,
#ts-waffle-button
{
    display: none;
}

.message-body {
    /* border: 1px solid blue;*/
}
.message-body-container {
    /* border: 1px solid red; */
}

.has-reactions .top-row-text-container {
    margin-bottom: 18px;
}

li.reaction-container {
    display: inline-block;
    padding: 0px;
    width: 45px;
    border: 1px solid #ccc;
    border-radius: 40px;
    margin: 8px;
}

li.reaction-container > div {
    margin: 5px;
}

li.reaction-container:hover {
    background: #ccc;
}

/* mention */
.message-body-content {
    padding: 3px 0;
}
.at-mentions-focus {
    border: 1px solid;
    border-radius: 4px;
    padding: 0 4px;
    margin: 1px;
}
.at-mentions-focus:hover {
    text-decoration: none;
    background: #ccc;
}

/* input */
.cke_contents span[itemtype="http://schema.skype.com/Mention"] {
    border: 1px solid;
    border-radius: 4px;
    margin: 0 2px;
    padding: 1px 3px
}