Use CSS File in include file without using L-INK

hi, 

we're using PHP File in whiche we include some files with e.g. navigation contenten and footer.

include ($cfgPath['elements'].'standard1.inc.php');
include ('./my-content.inc.php');
include ($cfgPath['elements'].'standard2.inc.php');

If I working on my-content.inc.php, I don't have all the style from our stylesheet, whiche will be includes in standard1.inc.php. In earlier versions of DW you could hook up a desing CSS file, which was uses in DW to show the propper design. In the current version I don't see a way to use the stylesheet without linking it with 

<link href="/lib/bootstrap-4.6.0/css/custom_main.min.css" rel="stylesheet" type="text/css">

And solutions or ideas?

TXS for yxour help.