Hi!
I found a site with a mobile browser detection code, please have a look codes:

if ($ENV{'HTTP_USER_AGENT'} =~ m/(android|bb\d+|meego).+mobile|avantgo|bada\/|blackberry|blazer|compal|elaine|f
ennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|midp|mmp|mobile.+firefox|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up\.(browser
|link)|vodafone|wap|windows ce|xda|xiino/i || substr($ENV{'HTTP_USER_AGENT'}, 0, 4) =~ m/1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s\-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as
(te|us)|attw|au(di|\-m|r |s |do(c|p)o|ds(12|\-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(\-|_)|g1 u|g560|gene|gf\-5|g\-mo|go(\.w|od)|gr(ad|un)|haie|hcit|hd\-(m|p|t)|hei\-|hi(pt|t
a)|hp( i|ip)|hs\-c|ht(c(\-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i\-(20|go|ma)|i230|iac( |\-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|kej
i|kgt( |\/)|klon|kpt |kwc\-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|\-[a-w]ts)|mmef|mo(01|02|bi|de|do|t(\-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|\-([1-8]|60|\-[2-7]l|ar|b3|it|t5)|so(ft|ny)|sp(01|h\-|v\-|v |5[0-3]|\-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(\-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|yas\-|your|zeto|zte\-/i) {
 	print "defaultmobile.css";
 	exit;
 }
  

(function(a,b){if(/(android|bb\d+|meego).+mobile|avantgo|bada\/|black
berry|blazer|compal|elaine|fennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|midp|mmp|mobile.+firefox|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up\.(browser
|link)|vodafone|wap|windows ce|xda|xiino/i.test(a)||/1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s\-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as
(te|us)|attw|au(di|\-m|r |s |do(c|p)o|ds(12|\-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(\-|_)|g1 u|g560|gene|gf\-5|g\-mo|go(\.w|od)|gr(ad|un)|haie|hcit|hd\-(m|p|t)|hei\-|hi(pt|t
a)|hp( i|ip)|hs\-c|ht(c(\-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i\-(20|go|ma)|i230|iac( |\-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|kej
i|kgt( |\/)|klon|kpt |kwc\-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|\-[a-w]ts)|mmef|mo(01|02|bi|de|do|t(\-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|\-([1-8]|60|\-[2-7]l|ar|b3|it|t5)|so(ft|ny)|sp(01|h\-|v\-|v |5[0-3]|\-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(\-| )|webc|whit|wi(g  

Another site suggested using this code to automatically switch from:

<link rel="stylesheet" href="...../yabbfiles/Templates/Forum/default.css" type="text/css" /> 

to:

<link rel="stylesheet" href="...../yabbfiles/Templates/Forum/defaultmobile.css" type="text/css" /> 

Unfortunately, I did not find where you can insert and use this code. As I understand, <yabb style> is connected with the user's choice of theme.

Maybe there is an easy way to apply style switching from default.css to defaultmobile.css  in automatic mode?

As I see it, the most difficult thing in the mobile version of the forum is typing. The current way of writing a post is not very convenient. You need to tap in the topic field, then in the text field, tap on tags. At the same time, the virtual keyboard constantly pops up.
It would be ideal if, when creating a post, a virtual keyboard with an input field will immediately open as in instant messengers, and a preview will immediately open above. Sending a message on clicking the submit button next to a single-line input field
Dream...