Override Gallery.css in Magento 2

You have to override the customer module to customize the register template. You have to create the new custom module and inside that module's etc/module.xml file, you have to set:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Package_Module" setup_version="2.0.1">
        <sequence>
            <module name="Magento_Customer"/>
        </sequence>
    </module>
</config>

As you already figured out how to use the variables, I'll briefly explain the 2 ways recommended by Magento to override or extend the styles from a parent theme:

1. Simple way

• Extend:

  In your theme directory, create a web/css/source sub-directory. (You've already done this part)

  Create a _extend.less file there. According to the documentation: "Extending a theme using _extend.less is the simplest option when you are happy with everything the parent theme has, but want to add more styles."

• Override:

  Instead of creating the _extend.less file, you create a _theme.less file.

  In this case you need to copy all variables you need from the parent _theme.less, including those which will not be changed. Then make your changes.

  Drawback: "You need to monitor and manually update your files whenever the parent’s _theme.less is updated."

2. Structured way

• Extend

  This method lets you organize your code in a better way. Your changes will be structured. Instead of using a single _extend.less file to include all your changes, you create a extend file for each component from the Magento UI library you want to change.

  Say you want to extend styles from button and navigation components. In your theme directory create 2 files:

  _buttons_extend.less and _navigation_extend.less, then add your changes for each component in the corresponding file.

  Then you create the _extend.less file adding this code:

  • @import '_buttons_extend.less';

  • @import '_navigation_extend.less';

• Override:

  In your theme, create a copy of the file corresponding to the UI component you want to change _buttons.less, _navigation.less etc This file will override the _buttons.less of the parent theme.

It is important to notice the difference between override and extend.

You can read more at: http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/css-guide/css_quick_guide_approach.html