Hi yiannismotsis
T3-assets is not the right place where make changes to css files, it is a temp folder used by t3- framework to store optimized files, so when you refresh cache it delete all your modifications
You should disable css optimization from your backend and then use the css files
Also to avoid update overrides over your customizations you should create a new file named custom.css in this folder /templates/ja-city_guide/css and add all your customizations to this file.
Please look at the T3 docs HERE
Templates customization isolation HERE
How to use custom.css file HERE
Regards