What's the best way to change font color to white for dark mode in iOS and Android devices for gmail and outlook?

Question

I need to display blue color text #243De9 in normal mode and white color in dark mode devices for gmail and outlook in iphone and android, but it's not working, after giving the specific CSS. please confirm how to proceed.

Normal mode is working fine, but for darkmode white color text need to display which is not coming.

@supports (-webkit-touch-callout: none) {
     /* CSS specific to iOS devices */ 
   .mobiledarkios {
     color:  #fff !important;
       }
}

@supports (-webkit-touch-callout: none) {
     /* CSS specific to android devices */ 
   .mobiledark {
  color:  #fff !important;
       font-family:Mulish,sans-serif;
     font-size:24px;
     font-style:normal;
     font-weight:900;
     letter-spacing:-0.5px;
  }
} 

 <td align="left" valign="top"><table bgcolor="#ffffff" cellpadding="0" cellspacing="0" style="font-size : 14px;font-family : Mulish, sans-serif;line-height: 150%;Letter-spacing: 0.5px;color : #000000;font-weight:400;" width="100%">
     
  <tr>
       <td class="mobiledarkios" style="color:#243De9 ;font-family:Mulish,sans-serif;font-size:24px;font-style:normal;font-weight:900;
    letter-spacing:-0.5px;">
         Hi&nbsp;%%=IIF(LowerCase(FirstName)=='unknown','there',FirstName)=%%</td>
  </tr><br>