domir21 Hi
It belongs to different parts, maybe style for modules / sections on that page. You can use inspect element in your web browser. What I usually do is hover on any HTML element (i.e. button, tag, text, …) you want and right click on it and then click on inspect element, it will show you which CSS/LESS class it is using and which CSS/LESS file and the path to this element on the right side. then you can go to the file and replace it with new CSS/LESS you change.
If you want to change in CSS, just add your custom CSS code into the file: root/templates/ja_smallbiz (your default template)/css/custom.css
In case that you want to customize LESS file, open the file: /templates/ja_smallbiz/less/variables.less and change the variable you wish.
For example, to change the background of the body color on website, you look for this code:
@body-bg: #f1e8df;
change to the color you want. Once done, you go to Backend > Extensions > Templates > JA Smallbiz template style > General tab > and see the status of 'Development Mode' option. If it's ON, then after changing LESS file, just refresh front-end of your site to see the change.
In case, this dev mode is OFF, you will need to compile LESS to CSS (Remember to backup all CSS files first as when compiling, they will be overwritten).