Ghost/ghost/admin/app/components/modal-upgrade-host-limit-custom-theme.hbs

<header class="modal-header" data-test-modal="delete-user">
    <h1>Upgrade to enable custom themes</h1>
</header>
<button class="close" title="Close" {{on "click" this.closeModal}}>{{svg-jar "close"}}<span class="hidden">Close</span></button>

<div class="modal-body">
    <p>
        {{#if this.model.limitErrorMessage}}
            {{html-safe this.model.limitErrorMessage}}
        {{else}}
            Your current plan only supports official themes. You can install them from the <a href="https://ghost.org/marketplace/">Ghost theme marketplace</a>.
        {{/if}}
    </p>
</div>

<div class="modal-footer">
    <button {{on "click" this.closeModal}} class="gh-btn" data-test-button="cancel-upgrade">
        <span>Cancel</span>
    </button>

    <button {{on "click" (action "upgrade")}} class="gh-btn gh-btn-green" data-test-button="upgrade-plan">
        <span>Upgrade</span>
    </button>
</div>