How to change color scheme of Keynote slide design/template?

If you want to change the appearance of all the slides:

1. While creating a new document, choose any template.
2. From the menubar, select View > Show Master Slides.
3. Select the layout you want to edit from the navigator pane on the left.
4. On the Toolbar, click on Inspector (“i” icon).
5. Choose the second tab icon (slide icon) in the Inspector window. Click on Appearance.
6. Here you can specify any background for the slide. It can be an image, a colour, a gradient or a tint.

If you just want to change the appearance of the current slide, follow steps 4 to 6.

To change the style of text for all the slides:

1. Follow steps 1 to 4.
2. Choose the capital “T” icon in the Inspector window.
3. Select the text whose colour, font, etc. you want to change.
4. In the inspector window, there are various properties like text colour, character spacing, etc. which you can change.
5. You can change the font from the toolbar.

To change the style of text for the current slide, just follow the steps without opening the master slide.

there's a simpler answer. choose whatever slides you want to use, go to view> lightable, then choose command A (select All), go to Format on the right and edit background as you want.