Please help us help you and fill where relevant:
Your LimeSurvey version: Version 6.4.4
Own server or LimeSurvey hosting: LimeSurvey Cloud 
Survey theme/template: Vanilla
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Hi there,
I am using "LimeSurvey Cloud Version 6.4.4" and I have noticed that when I import a survey where the template does not exist or is not compatible with the version of Lime, it uses a basic template that is not one of the default templates (as I understand it). I would like to create a new template using this model as a starting point and then make some modifications. 
Can anyone give me some advice on how to replicate this template as it is not one of the templates available for editing.
Thanks in advance!

manual.limesurvey.org/Theme_editor/en
manual.limesurvey.org/Global_settings/en#General (Default template)

Hi Denis,
Thank you for your reply. I'm sorry but I didn't explain myself well, I know how to work with templates. What I don't know is where the standard template is located so that I can replicate it. If I want to modify a template, I make a copy and then modify it. But in this case I don't know how to copy it because it's not one of the available templates. I was hoping that someone could tell me how to copy this template, which is not visible to the naked eye.

I don't know what you mean by "standard template". As far as I know, the "default" theme for LS6 is fruity_twentythree. This can be extended in the theme editor and then you can set the new theme as default in Configuration --> Global --> General and/or Configuration --> Global survey --> General settings.

Once that is set, any imported survey with missing or unavailable themes will initially use the extended theme.