function Set-SPOTheme{
    Param(
       [Parameter(Mandatory=$true)]$ctx,
       [Parameter(Mandatory=$true)]$spColorUrl,
       [Parameter(Mandatory=$true)]$shareGenerated,
       [string]$spFontUrl,
       [string]$backGroudUrl
    )
    try{
        write-host "Applying the theme to the site. " -NoNewline

        $spFontUrlAux = Out-Null #Needs to be set to Out-Null is empty
        If(![string]::IsNullOrEmpty($spFontUrl)){
            $spFontUrlAux = $spFontUrl
        } else {

        }

        $backGroudUrlAux = Out-Null #Needs to be set to Out-Null is empty
        If(![string]::IsNullOrEmpty($backGroudUrl)) {
            $backGroudUrlAux = $backGroudUrl
        }

        $web = $ctx.Web
        $web.ApplyTheme($spColorUrl, $spFontUrlAux, $backGroudUrlAux, $shareGenerated)
        $ctx.Load($web)
        $ctx.executeQuery()
        write-host " done." -ForegroundColor Green
    }
    catch{
        write-host "Error While applying the theme $($_.Exception.Message)" -foregroundcolor red
    }

}

Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"

$siteUrl = Read-Host -Prompt "Provide the URL"
$adminUsername = Read-Host -Prompt ("Provide user for {0}" -f $siteUrl)
$secureAdminPassword = Read-Host -Prompt ("Provide password for {0}" -f $adminUsername) -AsSecureString

$credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($adminUsername, $secureAdminPassword)
$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($siteUrl)
$ctx.Credentials = $credentials

#Sets only the Theme Colours
Set-SPOTheme $ctx "/_catalogs/theme/15/palette010.spcolor"

#Sets the Theme Colours and the fonts
Set-SPOTheme $ctx "/_catalogs/theme/15/palette015.spcolor" $true "/_catalogs/theme/15/fontscheme001.spfont"

#Sets the Theme Colours and the Image Background
Set-SPOTheme $ctx "/_catalogs/theme/15/palette010.spcolor" $true $null "/_layouts/15/images/image_bg006.jpg"

#Sets the Theme Colours, the fonts and the Image Background
Set-SPOTheme $ctx "/_catalogs/theme/15/palette015.spcolor" $true "/_catalogs/theme/15/fontscheme003.spfont" "/_layouts/15/images/image_bg005.jpg"