Visit the Slides Carnival website and check out all the beautiful and practical Google Slides design themes they offer free of charge. When you find one you like, you can click on it to bring up a more detailed page about that theme. Now, Slides Carnival does have some advertising on their page, which I agree is no fun, however it enables them to offer us free themes, so we just learn to navigate around it. When you want to download the theme, be sure you are clicking on the orange "Use this presentation template" button and not a download button of any kind.
When you click that button it will take you to a view only Slides presentation. You can then go to the File menu and choose "Make a copy".
It will prompt you to name the copy. I like to name it something that reminds me of what it looks like, so when I want to use it later, I can find it easily. I also add the words "presentation theme" to the name. I organize all of my presentation theme files into a folder in my Google Drive because it makes it easier to find the theme I want later.
Once you have your copy, you can close the window and browse for more themes. When you are ready to use that theme in a presentation, open a new slide show and click on the "Change theme" button across the top. At the bottom right hand corner of the panel that opens will be an "Import theme" button.
Type in the words "presentation theme" and all of the slide shows you have named with "presentation theme" in the filename will pop up. Choose the one you want and click, "Import theme".
Now you can build your presentation using that theme. When you click on the "Layout" button across the top of the page, you can choose which layout you'd like to use on your slide. You can also click on the little plus above the slide list. A short cut is to click the tiny down arrow instead of the plus because it lets you choose which type of slide you'd like to add.
Happy slide building!