The header regions styles/formatting of the theme are located at the beginning of the style.css file that came with the theme. You can easily find the header regions section in style.css by looking for the notation:

/******************/
/*HEADER REGIONS */
/******************/

The header region section of the style.css file controls most of the styling and formatting of the header regions which includes the site logo, site name, site slogan, search box, and the various other header block regions that the theme may have to offer.

The site’s name, the name of your web site or organization, is wrapped in a H1 tag and its styling/formatting can be adjusted by changing the properties of the H1 selector in the header regions section of style.css; the H1 selector in style.css can be found by looking for the notation /* site name */

Here, for example, you can change the font size, font type, and line height. However, the color of the site name is dependent on a different H1 selector (h1 a) because the site name in Drupal is usually a link to the home page. If you wish to change the color of the site name, please look for the following selectors in the style.css: h1 a (H1 anchor) which will be notated as /* site name link */ and usually immediately follows the previously above-mentioned site name H1 selector.

Also, controlled in the HEADER REGIONS section of the theme’s style.css file are the header block regions which may be available depending on the theme. Adjusting the height of the different header regions can be done here. Look for the header region you wish to adjust – header-top, header-first, header-middle, or header-last – add the desired height (in pixels) and save the style.css. Changing the width of the header regions is a bit more tricky and is probably best to be avoided. Most themes’ overall layout is set as a fixed width and attempting to modify the header regions’ width may cause the theme’s layout to break.