Any help is much appreciated.

Dear Support Team, I have uploaded my custom email signature that is made with HTML-CSS to Mozilla Thunderbird (as shown on the official documentation), expecting to embed the signature as it is showing in the design, but this hasn't worked properly. From my side as the sender, the email signature was missing the logo, QR code image, and social icons (all the images shown in the attached photos). And from the recipient side, the email signature is not showing the CSS styles at all for no significant reason. As a new subscriber to this mailing system, I need help from your end to resolve this issue, and I'm very sure you will make it for me. Looking forward to hearing from you as soon as possible. Thank you

Dear Support Team, I have uploaded my custom email signature that is made with HTML-CSS to Mozilla Thunderbird (as shown on the official documentation), expecting to embed the signature as it is showing in the design, but this hasn't worked properly. From my side as the sender, the email signature was missing the logo, QR code image, and social icons (all the images shown in the attached photos). And from the recipient side, the email signature is not showing the CSS styles at all for no significant reason. As a new subscriber to this mailing system, I need help from your end to resolve this issue, and I'm very sure you will make it for me. Looking forward to hearing from you as soon as possible. Thank you

One of the significant reason CSS does not show up to recipients is it is not well supported in email clients.

Inline CSS appears to generally work. See https://www.campaignmonitor.com/css/style-element/style-in-body/ and https://www.campaignmonitor.com/css/style-element/style-in-head/

Then there is the issue when using images in HTML that is not generated in Thunderbird is that the HTML generally links to a file on the local hard disk that is not available to the recipient and the moz-do-not-send is not correctly set in the HTML code.