Apply Theme in SharePoint 2013 using PowerShell

Themes are the quick and easiest way to apply branding to SharePoint sites. In SharePoint 2013 Themes got changed and redesigned as "Composed Look" feature. As it sounds "Composed" look, it simply defines theming by combining design elements: Master page - AKA site layout, Color theme, Font schemes and background images. Read more: Composed Looks Feature in SharePoint

Apply composed look programmatically in SharePoint 2013  
If you want to uniformly apply the composed look which comprises of Master Page, Image file, Font schema to all sites, you can utilize PowerShell.

Add-PSSnapin Microsoft.SharePoint.Powershell -ErrorAction SilentlyContinue

#Variables for processing
$WebURL="http://portal.crescent.com/"
$ColorPaletteURL="/_catalogs/theme/15/Palette015.spcolor"
$FontSchemeURL="/_catalogs/theme/15/fontscheme005.spfont"
$BackgroundImgURL="/_catalogs/theme/15/Tulips.jpg"

#Get the Web
$web = Get-SPWeb $WebURL

#Apply theme
$web.ApplyTheme($ColorPaletteURL, $FontSchemeURL, $BackgroundImgURL, $true)
$web.Update()

This applies the theme for given web.