Create a simple theme with a base of main theme

app/design/frontend/Leela/MyTheme/theme.xml

<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
    <title>Leela Theme</title>
    <parent>Magento/luma</parent>
</theme>

app/design/frontend/Leela/MyTheme/registration.php

<?php
use \Magento\Framework\Component\ComponentRegistrar;

ComponentRegistrar::register(
    ComponentRegistrar::THEME,
    'frontend/Leela/MyTheme',
    __DIR__
);

You can skip this step, and run Step 4 if you just want to register and see it in admin tool, If you want to make design changes to your theme you can create this folder structure and overwrite CSS by writing your own CSS in _extend.less. I have added some basic overwrite CSS

app/design/frontend/Leela/MyTheme/web/css/source/_extend.less

#search{

    border:red 1px solid;
}

#search_mini_form{

    .advanced {
        color:red;
    }

    .label{
        color:red;
    }

} 

Php bin/magento setup:upgrade

1. Login to your adminstration tool of magento
2. Go to Content -> Configuration
3. You should see either single store or multiple store, click on Edit of the store you want to apply your theme
4. You should see your theme name under drop down, in our case it ‘Leela Theme’ select it and click on apply
5. Run following commands

php bin/magento setup:static-content:deploy -f
php bin/magento cache:clean