Magento 2 make default system config value

please read first post make configure opitons for theme.

Find path config

We’ll make default system config value for our module. Please see file app/code/<vendor_name>/<module_name>/etc/adminhtml/system.xml

The path config is section/grounp/value

Make default system config

Now, we’ll create file app/code/<vendor_name>/<module_name>/etc/config.xml should look like:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
    <default>
        <themes><!-- section_id -->
            <general><!-- group_id -->
                <wellcome_text>Hi, Guest!</wellcome_text><!-- field_id -->
                <enable_topbanner>1</enable_topbanner><!-- field_id -->
                <background_image>default/forage-grass_landing-page_1600x300-1600x300_1.png</background_image><!-- field_id -->
            </general>
        </themes>
    </default>
</config>

This code will make default system config value when you install module.