We are using Same Porto Theme.
I recommend do not make changes in Porto Theme. Because they are upgrading their Versions. Once you upgrade to their latest version, your changes will be given.
So create your theme, by setting Porto as Parent.
magento\app\design\frontend\My\ThemeName\web\css\source\header.less
magento\app\design\frontend\My\ThemeName\web\css\source\footer.less
For how to create theme
http://devdocs.magento.com/guides/v2.1/frontend-dev-guide/themes/theme-create.html
Magento 2: How to override Luma Theme Home Page
magento\app\design\frontend\My\theme_name\theme.xml
<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
<title>My Porto Child</title>
<parent>Smartwave/porto</parent>
<media>
<preview_image>media/preview.png</preview_image>
</media>
</theme>
At the End Run below commands
php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy
php bin/magento indexer:reindex
php bin/magento cache:flush
chmod -R 777 var pub