The beautiful thing of WordPress (In my opinion) is that you can make almost any website and only have to make a theme and or plugin(s).
You can change a theme, delete things, extend etc. Or you can make a child theme, leaving the original theme intact and overwriting default behavior in a different place.
The rest of WordPress, the core, is there for you making theme or plugin building more easy (framework). You don’t have to touch these files.
Now back to your questions 🙂
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Most likely there is a default sidebar with some widgets. In the back-end go to “Appearance” > “Widgets” and there will be a sidebar (like default sidebar or something (depends on theme). When it’s empty you can add Widgets and you will see that the default sidebar is “overwritten” with your widgets. If not empty, you can remove the widget called “Pages”.
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In your theme folder you have to look at the template representing your front-page. This can be front-page.php, home.php page.php or index.php. Depending on which one exists (How that works: Template hierarchy). Look for a function like
the_date()orthe_time(). -
The CSS is most likely in style.css in the root of your theme folder. Sometimes a theme has a special CSS folder.
That said, you can change anything you want in a theme, but if you want to have the ability to update a theme, you can make a Child theme (See link above).