Hi edoardopost
This template is built with T3 framework, so in order to change style globally, you should change in LESS file.
I'm not sure which style and how you want to change so you can tell me more details, I will suggest further.
Normally, all variables are declared in this file:
/templates/sidewinder/less/variables.less
After changing less file, you will need to compile less to css to see the changes: https://www.t3-framework.org/documentation/bs3-configuration#compile-less
Regards