How to isolate email signature from global css in webmail

In our company, we recently started to use a new webmail interface. I created our email signature html code but when I add the email signature, I realized that webmail's global css effects my signature. Because of effecting, my signature looks differently.

The global css code adds borders to the table in my signature.

I tried many things to isolate my signature but I couldn't be successful. I tried the methods below;

- I added style='all:unset' and style='all:revert', but these method didn't work.
- I added style='isolation:isolate' , this didn't work either.

I am giving you a simple version of my signature, which looks not like the way I want.

<table id="main_div" style="isolation: isolate;" class="testa">
<tr style="isolation: isolate;">
    <td style="isolation: isolate;">
        <img src="https://www.borselboru.com/email/logo_black.png" width="130"  style="max-width: 130px;isolation: isolate;">      
    </td>
    <td style="isolation: isolate;">
        <h3 style="isolation: isolate;">
            <span style="isolation: isolate;">ARİF YILMAZ</span>
            <span style="isolation: isolate;">SATIŞ SORUMLUSU</span>
            <span style="isolation: isolate;">BORSEL BORU TİC. LTD. ŞTİ.</span>
        </h3>
    </td>
</tr>