You could build your own bottom nav bar, use transparent background icons on buttons, and set each button inside an arrangement. When button clicked, change the colour of the arrangement. Set all the buttons up in a list to handle click events so that only one button is coloured.
Download the Material Icons font in GitHub by clicking "View raw" and add it into App Inventor as an asset/media file.
Add one horizontal arrangement and 3 vertical arrangements (for 3 icons). Set all of their widths to fill parent, and place the 3 vertical arrangements inside the horizontal arrangement.
For 3 icons, you need 6 labels. Drag them into the designer like this.
Choose your favorite icons in Material Icons. Click on an icon, and copy their name, for example, you need "local_pizza" for the Local Pizza icon.
Customize the labels like this.
Set the custom font for the MyFonts extension to the Material Icons TTF font.