Here are the steps required to copy the Elemental theme to create a new theme called Test.

1. copy the elemental folder from the core themes directory into the application directory

copy from:
concrete\themes\

copy to:
application\themes\

2. rename the "elemental" folder to "test"

the theme folder name:
- must be lowercase
- cannot use spaces
- cannot use hyphens
- underscores can be used to separate words
Example:
my_theme

3. go to the Pages & Themes area in the dashboard

4. first error:

An unexpected error occurred. 
Cannot redeclare class Concrete\Theme\Elemental\PageTheme

5. first error fix: in page_theme.php

find:

namespace Concrete\Theme\Elemental;

change to:

namespace Application\Theme\Test;

when using folder names with underscores
- the underscore must be removed and replaced with CamelCase formatting
Example:
folder name: my_theme

namespace Application\Theme\MyTheme;

6. in your Test theme directory
version 5.7.4.2 or earlier
- open description.txt
- change "Elemental" to "Test"

if the folder was named my_theme
- change "Elemental" to "My Theme"

version 5.7.5 or later
- open page_theme.php
- look for getThemeName()

public function getThemeName()
{
    return t('Elemental');
}

change "Elemental" to "Test"

public function getThemeName()
{
    return t('Test');
}

7. install the Test theme

8. activate the Test theme

9. return to website

10. second error:

An unexpected error occurred.
File `../../../css/build/core/include/mixins.less` not found. in main.less

11. second error fix: in main.less (application\themes\test\css\main.less)

find:

@import "../../../css/build/core/include/mixins.less";

change to:

@import "../../../../concrete/css/build/core/include/mixins.less";

12. return to website

13. the site works and is now using a copy of Elemental called Test