When you apply a theme all the elements of the theme will be applied, fonts, background, colours etc...
But what if you need a totally blank slide in the middle of your presentation?
Here's a sample of a blank slide in one of the built-in themes in PPT. There is a graphic and a gradient fill.
Insert a slide, preferably one with the layout blank. Right-click on the slide background, choose Format background. In the task pane check Hide background graphics. Any graphics or logos from the master or layout will be hidden.That maybe enough to get a blank slide.
What if the background does not change to white? Then the graphics have been added to the background of the master itself, instead of inserting them on top of the master. Or the background of the master has been created with a Gradient or solid colour fill.
Here's an example of a gradient.
In that case you might have to use the radio button Solid fill and then apply the colour white next to Color in the same dialogue.
If you need several slides with that background you could create a new custom layout.
Go to View then click Slide master, the click the Insert Layout button, give the layout a representative name (right-click layout and choose Rename layout), and hide the background graphics. In addition you may have to change the background colour to a solid colour.
