Need a gradient? https://cssgradient.io/ (Also has color pickers, palettes, and swatches)
Need a textured background? https://coolbackgrounds.io/
Specifically a triangle textured background? https://trianglify.io/
Or how about moving particles? https://marcbruederlin.github.io/particles.js/
Or wallpaper photos? https://unsplash.com/
Never have a boring background.