Theme settings is where the global setting for the Shopify theme are specified. These include colours, fonts, favicon and social media links.  These are:

Colours

This section specifies the colours for areas of the sites such as text and buttons.  Colours can be selected from a pallet or defined using  hex code.

If you want to know the hex code for your image, you can use this tool: https://vidsbee.com/ColorPick/

Typography

This is where the fonts for the site are set.  Recommended to select system fonts as these will load quicker.

Styles

Accent colour selections for sale and sold out site badges.  The accent colour are set in the colour settings

Layout

Set the maximum width for the site.  Shopify sites are responsive so will adjust to screen size up to a maximum size.  Recommended to choose the largest size (1600px) to optimise how the site will look on bigger screens

Currency

Select whether the three letter currency code is seen after prices

e.g. £2.99 GBP or just £2.99

Search input

Select whether the site search displays suggested products (recommended).

Social media

Enter social media account for use in footer.

Favicon

A favicon is a small image that represents the site in the browser. They are 32×32 pixels. 

Quick favicon creator: https://favicon.io/

Checkout

The checkout is the same across all Shopify sites and so is edited in the theme settings and not in the sites theme.  The layout cannot be updated, but the colours and background images can be edited.