The button has a default font weight of --o-font-weight-7 which is a bold text. You can change this value by using the following CSS custom properties:
--o-font-weight-7
--o_text-font-weight
.o-btn { --o_text-font-weight: var(--o-font-weight-4);}
.o-btn-thin { --o_text-font-weight: var(--o-font-weight-1);}