Create Gradient Fill and apply to all slides

From Keynote help:
To create a new master slide:

1)Click View in the toolbar and choose Show Master Slides (or choose View > Show Master Slides) to open the master slide organizer.

2) In the master slide organizer (the upper portion of the slide organizer), select the master slide layout that most closely resembles the new one you want to create.

3) Click New in the toolbar (or choose Slide > New Master Slide).

4) Double-click the name of the new master slide and type a name.

5) Make your design changes to the master slide.

6) To add elements to the new master, open the Master Slide Inspector, click Appearance, and select the elements you want to add.

7) To allow the background element on the master slide to layer with objects that you place on the slide canvas, select the checkbox labeled "Allow objects on slides to layer with master."

8) To set a background color, gradient, or image, use the Background pop-up menu and select colors in the color wells.

9) To set a default transition for all slides that use this master, click Transition, choose an option from the Effect pop-up menu, and then set its direction and duration.

For example say I design a keynote and would like to use a gradient fill (the same) on every slide. How do I do this and replicate it on each slide in a way that I can change it quickly if I need to

Instead of putting the background on each individual slide, you put in on the few masters that those slides use. Then if you need to change the background, you change it on the masters, and not the individual slides.

For example, create a new presentation using the Gradient theme. Under the View menu, choose Navigator, and Show Master Slides. You should now see the main slide in the large window, the slide thumbnails at the bottom of the left vertical area, and the Master slides at the top left. Each slide you add uses one of the Master slides as its basis, which will be checked in the Master slide list.

Now, add several slides. These will all likely use the default "Title & Bullets" Master. Now, select the Title & Bullets Master, in the Master slide list. You can now edit this Master, and any change you make to it will be reflected in all the slides that use that Master. So, with the Master selected, go to the Inspector, choose the Master Slide palette, and then the Appearance tab. At the bottom of the palette you will see a section for Background. If you change this, you will change the background of the Title & Bullet Master, and thus of every slide using that Master.

So, to change the background of all the slides in a presentation, just change the backgrounds of the Masters used in the presentation. Note that, for this to work, you can't have set the backgrounds on the individual slides.

Does this help?

There are a few ways to get a consistent gradient across masters. One is to use an image for the background. If you create the gradient as an image in a graphics package, and then use that image as the background, you can get consistency that way. Alternatively, you can create the gradient you want in Keynote, then use the screen-capture function (Command-Shift-4) to select and take a picture of that slide background, which you can then use for all the master backgrounds.

A different approach is to use the Colors Palette to "save" the colours that you use in your gradient. First off, create your master gradient in Keynote, using the Master Slide Appearance palette. Then, making sure that the Colors Palette is visible, click one of the gradient colours in the Master Slide Apperance palette. This will cause this colour to appear in the main colour area at the top of the Color Palette.

Now look at the bottom of your Colour Palette -- you may see a grid of squares, and some squares may be coloured. If you don't see this grid, drag the small circular dimple at the bottom center of the Colour Palette to reveal this drawer. This drawer is where you can "store" commonly-used colours -- simply drag from the main colour area in the Color Palette to one of the white squares, and it will take on the colour you've dragged. Now, anytime you click on this square, the main colour area of the Color Palette will take on that colour, and any colour well that is selected (for example, in the Slide Appearance palette) will therefore also take that colour.

So, you can effectively create the gradient you want using two specified colours, and then save those colours so that you can set the other Master gradients to those specific hues.

If the blue background is on the masters for the slides, then all you have to do is change the master backgrounds, and all the slides that use that master will have their backgrounds changed.