Updates you made will work fine for the RTE design but it will not work with the HTML tab because if you will see the HTML part of it, you will see RTE design has its own iFrame with styles in it and HTML tab is a simple text area. Sitecore uses these UI controls from Telerik and if you want to apply the style on HTML tab as well then the easy way is to edit the EditorPage.aspx
file under sitecore\shell\Controls\Rich Text Editor
. you can open this file in any text editor and add the style in it as below -
.RadForm_Default.rfdTextarea textarea.reTextArea {
font-size:20px;
}
make sure to maintain the hierarchy for this. reTextArea
is class of the textarea tag in the editor. for refrence see below image -