Add theme 13 for the calculator

[alanboy]

Requirement: Create a new template

Open calculators.js find line 273 and find this code:

    // TODO: Add theme 13 - To fix this:
    //   - Change "theme name" to a catchy name (use your name or alias)
    //   - Change the colors blue and red with colors of your choice.
    //   - Uncomment the call to addOption (remove the `//`)
    // addOption("theme name", "linear-gradient(blue, red)", "red", "red");

As the comment suggests:

• Change the theme name to something meaningful
• Change the colors to those of your liking

Final result should look like this:

    // TODO: Add theme 13 - To fix this:
    //   - Change "theme name" to a catchy name (use your name or alias)
    //   - Change the colors blue and red with colors of your choice.
    //   - Uncomment the call to addOption (remove the `//`)
    addOption("alango theme", "linear-gradient(red, green)", "blue", "white");