So the first thing about making an aesthetic backstory is finding what aesthetic you want. There are a bunch, but a few common ones are:

Dark - Normally black text, sometimes in bold and italic, with black/dark red pictures.



Soft - Kinda just like the name, pale colors and pale-themed pictures. Commonly with Japanese quotes and/or pictures. It's also normally just pale blues, pinks, yellows and/or purples.



Colored - One color that the whole backstory is kinda based off. You could have red, orange, yellow, green, blue, purple, etc. etc, or a mix of colors! Three is the maximum, anything else can look like a whole different aesthetic or sometimes rainbow. The one below is a purple and yellow aesthetic, but it's just an example!



Vintage - It's kinda like a 70's/80's/90's themed aesthetic.



Art - It's just kinda an artsy, painting based aesthetic.



Baddie - Idk. This one lowkey confuses me.



Golden Hour - One of my favorites! It's so pretty. It's like a bunch of golden, kinda yellow-y themed pictures.



Fandom - There isn't really a color for this, it's kinda just in the pictures and quotes. The one below is a Hamilton themed moodboard.



There are probably a bunch more themes, but I'm kinda lazy lol.

You can also look up moodboards to fit the themes!