Did you try changing the Preview font Preferences in Sigil Preferences under the Appearance, the use the Preview Tab, just like in your picture? Try changing all fonts in that setting.
The default epub when opened has no text in it to display in Preview. Try typing somethind into codeview inside the p tag. Then try changing the fonts in Sigil Preferences.
And finally try editing the css used in the epub shown in your original photo and removing the following entry completely from it as it sets a font variant that may not exist in the chosen font for Preview.
Code:
body {
margin-left: 10%;
margin-right: 10%;
text-align: justify;
font-kerning:normal;
font-variant: common-ligatures oldstyle-nums proportional-nums;
font-feature-settings: "kern", "liga", "clig", "onum", "pnum";
font-family: serif;
font-size: 1em;
line-height: 1.5;
}
Please let me know what you find.