While HTML defines the structure and content of your web page, CSS which stands for Cascading Style Sheet, tells your web browser how to visually present the page's appearance, for example, colors and fonts:
https://www.tutorialspoint.com/css/what_is_css.htm
Even basic webpages will have a CSS style sheet, usually located at .css, in which the page's appearance will be defined by three sets: selectors, values and properties, handled around curly brackets { }.
For example, in the following CSS sheet:
.h5 class {
font-size: 12px;
color: blue;
}
- The selectors 'h5' and 'class' are telling the browser which bit of the page to apply changes to.
- The properties inside the curly bracket, 'color' and 'font-size' tell the browser the elements to change.
- The values '12px' and 'blue' tell the browser what style to apply to the selected parts.
CSS gets quite difficult and complex quickly, so it's important you have a web developer who understands how to work competently with CSS to provide the best visual outcome for your web application.