Image manipulation software
GIMP: https://www.gimp.org/downloads/
Free icons (with attribution to author)
https://www.flaticon.com
https://icons8.com
https://www.iconfinder.com
https://materialdesignicons.com/
https://material.io/resources/icons
Free backdrops (with attribution to author)
https://unsplash.com/
CSS Frameworks
https://material-ui.com/
https://bulma.io/
Colors
https://material.io/resources/color