Add styles directly to a block
Now I need to open the HTML editor and find the element then edit the style attributes. I want to edit it directly, under the "class name" field at the left side.
Also, I hope I can add classname and edit the style of the selected text, not only font size or color.

What you’re describing would best be done using the CSS editor in conjunction with the class name option, rather than adding styles directly to the HTML element.
More info about the CSS editor: http://help.slides.com/knowledgebase/articles/253052-css-editor-pro-
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Michael Lamparski commented
I feel there are advantages to having this regardless. E.g. many things (like groups) don't have scale or rotate settings and require a CSS transform. So, say you have a bunch of groups and you want to give each one a different rotation. Doing this with classes is tedious and pollutes the deck CSS sheet.