See https://www.woltlab.com/community/thread/301220-change-icon-in-admin-panel/
<!-- content -->
<acpmenuitem name="wcf.acp.menu.link.content">
<showorder>3</showorder>
- <icon>file-lines</icon>
+ <icon solid="true">file-lines</icon>
</acpmenuitem>
<!-- cms -->
<acpmenuitem name="wcf.acp.menu.link.cms">
<!-- application -->
<acpmenuitem name="wcf.acp.menu.link.application">
<showorder>4</showorder>
- <icon>comments</icon>
+ <icon solid="true">comments</icon>
</acpmenuitem>
<!-- /application -->
<!-- customization -->