Google Sheets is an online collaborative spreadsheet from Google that tries to be a clone of Microsoft Excel.
Compared to Google Docs, there is very little style that can be set for the current document in Sheets.
One solution seems to be to choose Format -> Theme and then customize the default theme. There are a mere 6 font choices in the theme: Arial, Comic Sans MS, Courier New, Georgia, Roboto and Verdana. There is no option provided to set the default font size. Apart from this, the theme allows you set some colors.
Another solution which only works on the current sheet is to select all cells by pressing
Ctrl + A and then setting the font, font size, colors and pretty much any style.
Compared to Google Docs, there does not seem to be any way to set the default style that is used for all subsequently created documents.
A partial solution, which is available only in Google Drive corporate accounts, is to set and create from a template: To do this, first create a new document and set the style you wish in it. Try to create a new document using New -> From template and in your organizations template gallery, use the Submit Template option to add the document you created earlier as a template. After this, whenever you choose to create a new document using New -> From Template, your template will be available as an option. The document created from your template will have the style you set.