Modified Links in Cover page (using CSS code) are not displayed as Buttons in visitor's screen

Site URL: https://breathe-move-connect.com/

I'd like to add a THIRD BUTTON in the initial COVER PAGE of my website (landing page). Since the standard customization does not provide me with this opportunity (it only allows 2 buttons), I created the 3 buttons as links in the standard customization and I put Custom CSS (in 'Design' page). The 3 links work normally, however they are not displayed as buttons on the screen but as links (When I press ‘save’ in my Cover page, I see them as buttons, but as a visitor I see them as links). Can anyone help me on this? Thank you!

The CSS code I used is:

.sqs-slice-navigation-content li {
  border-radius:50px;
  margin:20px;
  padding: 5px 19px 5px 19px;
  border:solid 2px #ffffff;
}

Hi. Custom CSS can't affect to Cover Page. Add this code to Cover Page Header

<style>
  .sqs-slice-navigation-content li {
  border-radius:50px;
  margin:20px;
  padding: 5px 19px 5px 19px;
  border:solid 2px #ffffff;
}
</style>

Thank you tuanphan. Unfortunately I use the personal plan and I do not have access to the Cover Page Header. If there is no other solution I will consider upgrading... Thanks again!