I hope you are well today and thank you for your question.
1) Is it possible to move the sidebar from right to left?
Yes you can achieve this by setting āWebsite Layout Optionsā to Left Sidebar in the Sparkling theme options as shown in the attached screenshot.
2) After reading this (https://colorlibsupport.com/t/border-colors/) I managed to remove all the borders but the sidebarās top one. How could I make it invisible too?
You can try removing it by adding the following CSS code in the Custom CSS option of your theme on the below path.
Admin Area -> Apperance -> Theme Options -> Other -> Custom CSS
#secondary .well {
border: 0;
}
An extra question: how can I undo justification all at once through CSS in Sparkling?
You can try doing this using the following CSS code.