If you use Google Slides you will know that the number of theme choices is limited. However, you can design your own theme! Or you can simply go to slidesmania.com and Paula creates free themes for you. While I use Slidesmania a lot, I often want to design and customize my own Google Slides.
1. Theme Builder
You are wanting to customize the theme or build your own theme. Your first step is to use the View menu and then choose “Theme builder.”

2. Theme
While in theme builder you might notice that it says the theme is “Simple Light.” You want to change this to make it your own theme! Notice the selection of layouts and above that layout is a tile called “Theme” (formerly the master.) First, click on the Theme above layouts. Second, click on “Rename” in the middle above the large version of the theme slide.

The theme slide controls the entire Google Slides. Change the font colors and sizes on this theme layout to make universal changes to ALL the layouts. Adding an image onto the theme layout will cause that image to appear on ALL slides.
3. Select Theme Colors
It is important that you define your theme colors. When you use theme colors in your presentation, you can modify the theme colors and this universally makes changes to your Google Slides.

Tip: Modify Colors on SlidesMania
After downloading a Google Slides from SlidesMania you may want to customize the colors to match your school colors. Change the colors in the theme builder to make global changes to the entire presentation!
In the sidebar, click on “Choose a theme color.”

4. Use the Theme Colors
I can not emphasize this enough. Use the theme colors. In the sidebar under colors you clicked on each of the options and chose a complimentary set of colors.

Add Shapes to the Layouts
There are 11 default layouts. You now customize these with shapes, images, and adjusting the placeholder textboxes. When adding shapes, click on the paint can in the toolbar. Notice that the theme name you chose is now listed instead of “Simple white.” Click on these color options as often as possible.

If you go back to the Theme Builder and change any of the colors on the theme, for example you would rather have hot pink instead of purple, then every spot where the theme color was used will now be hot pink instead.