On any sites, if Main Title Text (H1) filled under Settings --> Design Settings --> Homepage Layout:

This element will be displayed on the homepage with a default Font Weight of 600 (its property value):

This value can be overwritten by simply adding this CSS code to your websites Custom CSS/HEAD section:

.homepage_title h1 {
    font-weight: 100;

This section can be found under Settings --> Design Settings --> Custom CSS/HEAD section: 

This is how the homepage main title text looks after the code has been added: 

Additional Font Weight Property Values:

Defines normal characters. This is default
Defines thick characters
Defines thicker characters
Defines lighter characters
100 - 900
Defines from thin to thick characters. 400 is the same as normal, and 700 is the same as bold