how to add a underline under the img of the page header

[BetterLYY]

I want to add a underline for the header of ever page, and I have added a base64 img to the header. I try to the following code to add the underline, but it seems that the underline will not be took effect on the img, so how can I set the underline line under the image, thanks very much!

<div style="font-size: 9px; margin-left: 1cm; text-decoration: underline; display: block;"><img src="data:image/png;base64,...=" alt="" width=20/>&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbspend_of_the_header</div>

[BetterLYY]

is there any method to get the style of Markdown-pdf:header-template, so that I can customize it according to the demand.