Lighter FreeCAD Forum Userstyle. Install with Stylus extension. Tested on Firefox.

lighter_freecad_forum.user.css

/* ==UserStyle==
@name           Lighter FreeCAD Forum
@namespace      github.com/foosel
@version        1.0.2
@description    A lighter theme for the FreeCAD forum. Sans-serif instead of Fira-Sans, slightly more line-height, slightly less bold headlines, less read everywhere. Easier on the eyes.
@author         foosel
@homepageURL    https://gist.github.com/foosel/21fb760e94bae5782171ca49c799daf5
@updateURL      https://gist.githubusercontent.com/foosel/21fb760e94bae5782171ca49c799daf5/raw/lighter_freecad_forum.user.css
==/UserStyle== */

@-moz-document domain("forum.freecadweb.org") {
	body,
	.content {
		font-family: sans-serif !important;
	}

	.topictitle,
	.forumtitle {
		font-weight: 500 !important;
	}

	.content {
		line-height: 1.5 !important;
	}

	.icon-link + .list-inner > a.forumtitle,
	.icon-link + .list-inner > a.topictitle,
	.pm_read .topictitle,
	a.subforum.unread {
		color: #337ab7;
	}
}