Trying to change the font to Roboto Thin 100

I am trying to change font to Roboto Thin 100. Below code changes the font on sidebar but not inside editor. Is this correct CSS snippet?

ref:Google Fonts

/* Override font */

div {
    font-family: roboto;
    font-size: 1em;
    margin: 0px;
    padding: 0px;
}

Not tested, but from the source you linked (Google fonts) it’s more like

@import url('https://fonts.googleapis.com/css2?family=Roboto:wght@100&display=swap');

div {
  font-family: 'Roboto', sans-serif;
}

I’m very used to Workflowy default serif font. How can I change Remnote to looks like it?

It’s this font:

body {
  font-family: Georgia, Times, serif;
  font-weight: normal;
  color: rgb(42, 49, 53);
}

And this if you want to customize fontsize and spacing a bit:

.rem-text:not(.rem-header--1):not(.rem-header--2):not(.rem-header--3) {
  line-height: 20px;
  font-size: 16px;
}

Thanks so much. It worked. Does this download font everytime or just once from the url?

It downloads every time you start RemNote possibly minus some (browser) caching. It should also be possible to install the font on your device somehow but I don’t know how and how that would work with a font variant (Thin 100 in your case).