Sorry Jacky… anything that is custom based (theme customization) such as the kind of changes you are asking is not covered by theme support. Putting a search field where the main menu at the top requires a lot of custom coding and not something that can easily be written out as a step by step tutorial. This one you will have to try out yourself, but if you are a full time website builder as you say you are, then you should be able to figure it out.
Regarding adding the searchform.php to the style.css is definitely not correct because the searchform.php is a php file whereas the style.css is only css.
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