Change Comment's Default Formatting

You can insert comments to a cell in Excel. If you don’t like the format of it, you can change it by selecting the comment and right click, choose Format Comment option. But if you want to change the default format of the comment so that you need not change the format every time you insert a comment, follow the steps below.

Excel's comment uses Windows ToolTip settings for its own formatting. So, you have to change the ToolTip settings in order to set comment's default format. This is the sequence in my Windows XP. Similar commands should be available in other OS as well. Try any one of them.

Start > Control Panel > Appearance and Themes > Display (in category view)

Start > Control Panel > double click Display (in classic view)
Right click in any empty space on desktop > Properties

In the Display Properties dialogue box, click on Appearance tab and click Advanced button! In Advanced Appearance dialogue box, under Item dropdown, select Tooltip, adjust the background color, font color, font size, Italic, Bold properties etc here.

Changing Windows ToolTip format will effect all other Windows based programs too.
You may want to read how to insert picture background in comments.