magento-test/app/code/Magento/Theme/Test/Mftf/ActionGroup/AdminChangeStorefrontThemeActionGroup.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
 /**
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
-->

<actionGroups xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xsi:noNamespaceSchemaLocation="urn:magento:mftf:Test/etc/actionGroupSchema.xsd">
    <actionGroup name="AdminChangeStorefrontThemeActionGroup">
        <arguments>
            <argument name="theme" type="string"/>
            <argument name="scopeColumn" type="string" defaultValue="Store View"/>
            <argument name="scopeName" type="string" defaultValue="{{_defaultStore.name}}"/>
        </arguments>
        <amOnPage url="{{DesignConfigPage.url}}" stepKey="navigateToDesignConfigPage"/>
        <click selector="{{AdminDesignConfigSection.scopeEditLinkByName(scopeColumn, scopeName)}}" stepKey="editScopeConfig"/>
        <selectOption selector="{{AdminDesignConfigSection.appliedTheme}}" userInput="{{theme}}" stepKey="selectTheme"/>
        <click selector="{{AdminMainActionsSection.save}}" stepKey="clickSave"/>
        <waitForElementVisible selector="{{AdminMessagesSection.success}}" stepKey="waitForSuccessMessage"/>
        <see selector="{{AdminMessagesSection.success}}" userInput="You saved the configuration." stepKey="seeSuccessMessage"/>
    </actionGroup>
</actionGroups>