The ability to create a Keynote document using a specific theme is enabled by the inclusion of the document theme property of the document class.  (⬇ see below ) 

The Keynote Suite Document Class (excerpt)

The value of the document theme property  (⬆ see above )  is the theme currently used (or to be used) by the document in the display of its contents.

NOTE: The document theme property is now read and write and can be used to change the current theme of a Keynote document.

The script below demonstrates the use of the make command to create a new document with a theme chosen by the script user:

New Document with Chosen Theme
Open in Script Editor
01		tell application "Keynote"
02		activate
03		try
04		-- get theme names
05		set the themeNames to the name of every theme
06
07		-- prompt user to pick a theme
08		set thisThemeName to ¬
09		(choose from list themeNames with prompt "Choose theme:")
10		if thisThemeName is false then error number -128
11
12		-- convert resulting list into string: {"Black"} to "Black"
13		set thisThemeName to thisThemeName as string
14
15		-- create default document, styled with the chosen theme
16		make new document with properties ¬
17		{document theme:theme thisThemeName}
18
19		on error errorMessage number errorNumber
20		if errorNumber is not -128 then
21		display alert "THEME ISSUE" message errorMessage
22		end if
23		end try
24		end tell

The following script, a variation of the previous script, creates a new Keynote document using the application theme chosen by the script user:

New Document with Chosen Application Theme
Open in Script Editor
01		tell application "Keynote"
02		activate
03		try
04		-- get application theme names
05		set theseApplicationThemeNames to ¬
06		the name of every theme whose id of it begins with "Application/"
07
08		-- prompt user to pick a theme
09		set thisThemeName to ¬
10		(choose from list theseApplicationThemeNames with prompt "Choose a theme:")
11		if thisThemeName is false then error number -128
12
13		-- convert resulting list into a string: {"Black"} to "Black"
14		set thisThemeName to thisThemeName as string
15
16		-- create a default document, styled with the chosen theme
17		make new document with properties ¬
18		{document theme:theme thisThemeName}
19
20		on error errorMessage number errorNumber
21		if errorNumber is not -128 then
22		display alert "THEME ISSUE" message errorMessage
23		end if
24		end try
25		end tell

The following script is designed to create a new Keynote document using a specific user theme. If the user theme is not installed on the computer running the script, the script will prompt the script user to download the specified user theme from the company website, and will display the company-hosted website page containing the theme if the user desires.

Create Document with Specific User Theme
Open in Script Editor
01		tell application "Keynote"
02		activate
03		try
04		-- get application theme names
05		set theseUserThemeNames to ¬
06		the name of every theme whose id of it begins with "User/"
07
08		-- check for installation of user theme
09		set thisUserThemeName to "Official Corporate Theme"
10		if thisUserThemeName is not in theseUserThemeNames then
11		-- notify user of missing resource
12		set thisMessage to "The Keynote theme “" & thisUserThemeName & "” " & ¬
13		"is not installed on this computer." & return & return & ¬
14		"Download the theme from the company webiste."
15		display alert "MISSING THEME" message thisMessage ¬
16		buttons {"Download", "Cancel"} default button 2
17		if button returned of the result is "Download" then
18		open location "https://mycompanyname.com/communications/templates"
19		end if
20		else
21		-- create a default document, styled with the theme
22		make new document with properties ¬
23		{document theme:theme thisUserThemeName}
24		end if
25		on error errorMessage number errorNumber
26		if errorNumber is not -128 then
27		display alert ("ERROR " & errorNumber) as string message errorMessage
28		end if
29		end try
30		end tell