Developers, could you consider making some kind of configurable themes? E.g.
/*pregenerated part - on theme change, on output display change*/
.theme1 .main, .theme1 .footer {
position:.... width: ...: height....
}
/* user customizable part */
.theme1 .myUpdate {
...flex reorder...
}
<div class="theme1 myUpdate">
<div class="main">{mainContent()}</div>
<div class="footer">{footerContent()}</div>
</div>