@carlfriedrich my friend I sloved this problem
SOLVED 
The best way is next:
- download Prestashop 1.8 zip file to your desktop
- unpack this zip file in his directory (when unzip choose "Extract to..."
- open this folder then unpack prestahop zip file with option Extract to prestashop"
- open this folder Prestashop -> themes -> classic -> assets -> css and select all files and copy to your server in folder themes ->classic->assets->css (overvrite
- refresh BO and FO with ctrl F5
In css folder on server you have only 2 files but you need all files to be there. All files from zip file folder css (you will see some svg, ttf files and others)