Ghost/ghost/admin/app/templates/settings/design/index.hbs

<section class="gh-canvas gh-design">
    <GhCanvasHeader class="gh-canvas-header">
        <h2 class="gh-canvas-title">Site design</h2>
        <section class="view-actions">
            <div class="gh-select gh-design-page-preview-mode">
                <OneWaySelect
                    @value={{this.themeManagement.previewType}}
                    @options={{this.themeManagement.availablePreviewTypes}}
                    @optionValuePath="name"
                    @optionLabelPath="label"
                    @optionTargetPath="name"
                    @update={{this.themeManagement.setPreviewType}}
                />
                {{svg-jar "arrow-down-small"}}
            </div>

            <div class="gh-contentfilter gh-btn-group">
                <button type="button" class="gh-btn gh-design-preview-mode {{if this.isDesktopPreview "gh-btn-group-selected"}}" {{on "click" (fn this.setPreviewSize "desktop")}}><span>{{svg-jar "desktop"}}</span></button>
                <button type="button" class="gh-btn gh-design-preview-mode {{if this.isMobilePreview "gh-btn-group-selected"}}" {{on "click" (fn this.setPreviewSize "mobile")}}><span>{{svg-jar "mobile-phone"}}</span></button>
            </div>

            <GhTaskButton
                @buttonText="Save changes"
                @task={{this.saveTask}}
                @successText="Saved"
                @runningText="Saving"
                @class="gh-btn gh-btn-primary gh-btn-icon"
                data-test-button="save-settings"
            />
        </section>
    </GhCanvasHeader>

    <div class="view-container">
        {{!-- changes only classes between desktop/mobile to avoid re-rendering iframe contents --}}
        <div class="{{if this.isMobilePreview "gh-pe-mobile-container flex justify-center" "gh-browserpreview-previewcontainer"}}">
            {{#if this.isDesktopPreview}}
                <div class="gh-browserpreview-browser">
                    <div class="tabs">
                        <ul><li></li><li></li><li></li></ul>
                        <div>
                            {{#if this.settings.icon}}
                                <span class="favicon"><img src={{this.settings.icon}} alt="icon"></span>
                            {{else}}
                                <span class="favicon default">{{svg-jar "default-favicon"}}</span>
                            {{/if}}
                            <span class="site-title">{{this.settings.title}}</span>
                        </div>
                    </div>
                </div>
            {{/if}}
            <div class="{{if this.isMobilePreview "gh-pe-mobile-bezel" "gh-browserpreview-iframecontainer"}}">
                <div class="{{if this.isMobilePreview "gh-pe-mobile-screen"}}">
                    <GhHtmlIframe class={{if this.isMobilePreview "gh-post-preview-iframe" "site-frame"}} @html={{this.themeManagement.previewHtml}} />
                </div>
            </div>
        </div>
    </div>
</section>