Update: Use the new template designer to change font instead. Login to Dashboard > Layout and click the template designer at the top right corner of the page.
It is now very easy to change the fonts and colors of various things in your New Blogger blog. You don't need to tamper with the template. You can do it direct from the LAYOUT.
Sign into blogger.com (Dashboard). You will see a list of all your blogs. Click LAYOUT for the relevant blog. Then click the FONTS AND COLORS sub-tab. See screenshot below:
Say, you want to change the font for the Header. Scroll down till you see "Page Header Font". On the right, you will have the radio button to chose the font family and further to the right, you can change the font style to bold or italic. There are also linls to change the font size to "bigger" or "smaller"