Copying PowerPoint slides from one presentation to another can have disastrous format implications. Some survival advice.
- When saving/defining a new PowerPoint theme, stick to the suggested colour uses that PowerPoint suggests, i.e., text/background dark should be a dark colour for example. If you move slides across between templates in properly defined colour schemes, the damage will not be that big
- Make sure you copy slides into the file with the desired template and not the other way around. Sometimes this might require you to create a 1-slide presentation in your preferred template, and then copy the 35 other slides into it.
- There is a way to merge PowerPoint slides and keep their original formatting, see an old blog post on the subject.
- At the top left corner of the ribbon is a layout button that opens a drop down menu of slide formats that are present in the master. Use to to correct disasters.
Good luck!