Hi there!
After saving a theme I would like to use the same format of the words, including size, style, and color). Is this possible? I am not able to do that so far. After opening a new document with my saved theme will be a different (a standard TimesNewRoman-sh and a small size). Many thanks for your answers.
Unfortunately, it is not entirely clear if you mean host Writer (word processor) or Impress (presentation).
I hope you do not make your settings for the font and the other properties as hard-formatting.
You should open a blank document. In it you make all settings using stylesheets.
When you have finished everything, you save the document as a template, for example. You can save it as a default template, then it will always be opened when calling the Writer, with the settings made by you. Or you save it with a name of your choice, then you have to call it via New>Templates.
The documentation for LibreOffice can be found here.