Copy a slide from one deck into another and the software asks you a question, usually through a small paste-options control that people click past without reading. The question is which theme wins. (PowerPoint for Microsoft 365 desktop offers Use Destination Theme and Keep Source Formatting on paste; checked 2026-09-08.)
Keep Source Formatting imports the source master into the destination file. Now you've got two masters and two themes, and half the slides ignore your theme edits. Use Destination Theme re-maps what it can and leaves the rest. It's almost always the right choice, though it won't finish the job on its own.
Worked example: nine slides merged into a client deck
Harbor Point Capital, a synthetic US advisory firm, merged nine slides from Lantern Health Analytics, a synthetic portfolio company, into a client deck. Those nine slides carried 53 formatting-bearing elements. Sorting them by what actually resolved the collision gives a four-way split.
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| Resolution | Elements | Share |
|---|---|---|
| Handled by Use Destination Theme alone | 16 | 30.2% |
| Handled by remapping the slide to a destination layout, then Reset | 14 | 26.4% |
| Needed hand repair | 20 | 37.7% |
| Correctly left alone | 3 | 5.7% |
| Total | 53 | 100.0% |
One inheritance rule explains every row of that table. Learn the rule and you won't need to memorise four cases. An element re-maps automatically if and only if it arrived in a placeholder whose role exists in the destination layout set. Titles in title placeholders re-map, all six of them. Body in content placeholders mostly re-maps, five of seven. The two exceptions had been split into an arrangement the destination has no layout for. Titles in floating boxes can't re-map, because a floating box has no role to map from, so all three needed the remap-and-Reset treatment. Charts carry their own style and re-map only when their series were pointing at theme slots, which was true of one of the four. Images have no theme dependency at all, so all three were correctly left alone.
Is it cheaper to repair or rebuild the slides?
The economics favour repair here, and it's worth running the comparison before you start. Repair cost 41.00 minutes: twenty hand elements at ninety-five seconds each, plus fourteen remaps at forty seconds. Rebuilding the nine slides from scratch at six minutes a slide would have cost 54.00 minutes. Repair wins by 13.00 minutes, which is 4.56 minutes a slide against 6.00, a saving of 24.1 percent. Put the other way round, repairing all nine costs what rebuilding 6.83 of them would have cost.
That crossover is the number to carry, because it flips. Say the import had been three slides with the same element density. Repair would still have cost most of its fixed diagnosis time, rebuild would have cost eighteen minutes, and rebuilding would have been the better call. Repair pays on large imports of structured content and loses on small imports of messy content.
What should you do after the merge?
Finish the merge by consolidating. The file arrived with two masters carrying eleven and nine layouts, twenty in total. Consolidating to one master and the eight-role set removes twelve layouts, a 60.0 percent reduction, and it is the step people skip because the deck already looks right. It does look right. It'll stop looking right the next time somebody adds a slide and picks a layout from the wrong master.