Hi,
the plugin will not use your theme styling out of box as each theme is different and it is impossible to tell what CSS classes will the theme use to style various elements, so WPAdverts is using its own styling.
You can make it use your theme styling by adding a custom CSS code in wp-admin / Appearance / Customize / Additional CSS panel
body .adverts-button {
background: #3e69dc;
border: 0;
border-radius: 5em;
color: #fff;
display: inline-block;
font-size: 13px;
font-size: 0.8125rem;
font-weight: bold;
letter-spacing: 0.0625em;
line-height: 1.3847;
padding: 0.5625rem 1.5em;
text-transform: uppercase;
}
I am not sure if this will work properly on your website, it would be best if you could paste here a link to a page where i can see the button which is not displaying correctly (currently this page seems to be behind a paywall?), then i could double check the CSS code.