Improve Theme Customization section on the README

[vczb]

We should improve the Theme Customization section on here:

should improve this exemple as:

....
  Object.assign(theme.colors.primary, { medium: "#510763" });

  return (
    <SaguProvider theme={theme}>
....

---

and here:

  const customColors = {
    tertiary: {
      lighter: "#fb973a",
      light: "#e37c1d",
      medium: "#da710f",
      dark: "#9e4c01"
    }
  };

  Object.assign(theme.colors, customColors);
  
  return (
    <SaguProvider theme={theme}>

---

please, be smart... you need to keep the rest of the example and add the improvement, make sure that it is properly formatted etc... thanks

[KikiQianSun]

Hi, could you assign this issue to me, please?

[vczb]

closed by #121