If you would like to use a specific font throughout all your store pages, you can setup one by adding the following CSS rule in your custom theme in Ecwid control panel → System Settings → Design:

.ecwid div, .ecwid span, .ecwid a, .ecwid p {
font-family: NAME_OF_FONT !important;
}

where replace NAME_OF_FONT with a font, for example:

.ecwid div, .ecwid span, .ecwid a, .ecwid p {
font-family: verdana, arial, sans-serif !important;
}