I started work on a JavaScript way to do what you want late last night ( 😴 ) but didn't finish until this morning.
@tuanphan's method is a crafty use of CSS to get the job done! 👍
My alternate method attempts to address the fundamental issue with the site title, in that you can't address the individual words of the title with CSS.
Please see Site Title Each Word Span Tag Add.
The code is a base effect so you have to add CSS to achieve the final effect.
For your issue it would be something like the following.
#site-title span:nth-child( 2 ) {
font-weight : bold;
}