Opacity and Transparency are two important features by which you can allow your background comes up as a highlight. In Google Slides you can adjust and lower the opacity of an image, shapes, and text also.
Let us see how to change opacity in Google Slides for images, shapes, and text. Some more regarding Google Slides background
How to change the opacity of an image in Google Slides
Open Google Slides on your desktop computer
In the “Menu” bar click on the “Insert” tab then navigate to the “Image” option and add the image in Google Slides from various options.

After inserting the image, to enable the opacity option, right-click the image and choose “Format options” or in the menu bar click “Format options”.

A list of “Format options” will appear on the right side of the slides. To change the image opacity in Google Slides, under the “Adjustments” drag the “Transparency bar” to decrease or reduce the opacity. See below screenshot-

This will help you to lower the opacity of an image on Google Slides.
How to make a shape transparent in Google Slides
You may also have 2d or 3d shapes in Google Slides. To change the transparency of shape draw any shapes with the help of drawing, follow the below steps-
In the “Menu” bar click on the “Insert” option then go to Shape and navigate to the Shapes option, select and draw the shape in slides that you want to insert, and make it opaque.

After adding the shape in slides, under the menu bar click on the “Fill Color” icon, from there choose the “CUSTOM” option to select the color, and make it transparent.

Now lower the “TRANSPARENCY” slider to change opacity of shape and hit the OK button. See reference below-

Here for example we reduce the opacity of the heart shape with a Chrome icon as a background image. After changing the transparency of your shape it will look like the below image.

How to make text transparent in Google Slides
For changing the opacity of text you will need to insert the text via Word Art, follow the below steps-
In the “Menu” bar of Google Slides click to “Insert” then navigate and click to the “Word Art” option, write text in the box and adjust its height, width, boldness, font, etc.
Before changing Google Slides text transparency you will have to fill the text color by selecting the “Fill Color” option and selecting the “CUSTOM” from the color palette.

Now drag and lower the Transparency slider to reduce the opacity of text and hit the OK button. See how your text will look after lowering the opacity of the text.

Changing Google Slides opacity of image, shape, and text is easy on desktop as compared to a mobile app.