Go to your Bio/ Landing Page in edit mode and navigate to ‘Settings’, at the bottom of this page, enter the CSS code below within the ‘Custom CSS’ box (change your Colour Hexadecimal).
You can use the Colour Hexadecimal picker from this tool at https://www.color-hex.com. 

To customise the social links/ icons colour

a {
 color: #ffffff;
}

To customise the hyperlink colour of HTML block

Adding !important; will override system default colour - as an example, the hyperlink colour is red.

.item.mb-3 a
 color: red !important;
}

To customise button colour with mouse hover

Below example, the button color changes to black when the mouse hovers over it.,

.btn:hover {
   background-color: #000000 !important;
}

In fact, you can use the same Custom CSS box to do advanced customisation on your Bio/ Landing Page such as font size, themes, header styling etc.