If you want to modify the ui you can create your own theme or modify an existing theme. In the folder ..Content/Themes/.
You also need to download the source package and you need a SASS compiler to compiler your theme.
If you want to change the font you can do it easily by modifing the _variables.scss
$font-family-sans-serif: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji" !default;
$font-family-monospace: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace !default;
$font-family-base: $font-family-sans-serif !default;
and add whatever you want