metacitythemes
I have a problem with my current theme. When windows top border close to top panel, there's a transparent background at the top corner (please take a look at the attached image).
So, how could I :
Thanks in advance.
First you will need to find a theme with the buttons you want and install that theme.
You can find many OS X themes at gnome-look, search gtk3 theme in the type and osx in the description, though I will recommend you, based on your question, this nice flat Yosemite theme (Developers screenshot).
This should look really nice with the Arc theme, in my opinion.
Next install
sudo apt-get install gnome-tweak-tool
When it is finished start this app from the launcher or
gnome-tweak-tool
Browse to the Appearance section and you will see this
Change the window theme to change your window declarations and gtk3 for the rest of the theme.
Best Answer
To change the corners, you will have to edit the theme, or choose a different one. Also, you should ensure compositing is enabled, if you want rounded corners. You can do this by using compiz instead, or by running
gconf-editor, opening the/apps/metacity/generalitem in the tree, and enabling thecompositor_effectsoption in the list on the right. With that on, you should not see this artifact in the theme.