style your HTML with CSS

CSS treats each HTML element as if it appears inside its own box and uses rules to indicate how that element should look.
Rules are made up of selectors (that specify the elements the rule applies to) and declarations (that indicate what these elements should look like).
Different types of selectors allow you to target your rules at different elements.
Declarations are made up of two parts: the properties of the element that you want to change, and the values of those properties. For example, the font-family property sets the choice of font, and the value arial specifies Arial as the preferred typeface.
CSS rules usually appear in a separate document, although they may appear within an HTML page.

color with HTML

Color not only brings your site to life, but also helps convey the mood and evokes reactions.
There are three ways to specify colors in CSS: RGB values, hex codes, and color names.
Color pickers can help you find the color you want.
It is important to ensure that there is enough contrast between any text and the background color (otherwise people will not be able to read your content).
CSS3 has introduced an extra value for RGB colors to indicate opacity. It is known as RGBA.
CSS3 also allows you to specify colors as HSL values, with an optional opacity value. It is known as HSLA