There are various ways to change the color of the text, but there is also a systematic way for theme-based slides to change all the text colors altogether for both dark and light backgrounds.
Steps for how to change text color in theme-based slides.
- Go to the “Design” tab in your PowerPoint presentation slide.

- Then Under Design go to the variants section and click the corner arrow as shown in the picture.

- A list of options will appear, go to the color option which will open another list of color themes.

- Click the “Customize Colors” option from the bottom of the list of color themes.

- “Create New Theme Colors” window will appear with theme color options for Text/Background in your presentation. Select the text color you want for Dark and light background.

- Click the color box arrows and select color from the theme colors and It will set your background and text color accordingly.

- Click the save button from the bottom of the window

- Boom your desired color text will appear on the slide now.