If you have already downloaded the plugin file from a any website, you can upload it manually by following these steps:

  1. Log in to your WordPress dashboard.
  2. Navigate to "Plugins" in the left-hand menu and click on "Add New."
  3. Click on the "Upload Plugin" button at the top of the page.
  4. Click on the "Choose File" button and select the plugin file from your computer.
  5. Click the "Install Now" button to start the installation process.
  6. Wait for the installation to complete, and then click the "Activate" button to enable the plugin.

Installing a new theme in WordPress is also a simple process. Here are the steps you can follow:

  1. Log in to your WordPress dashboard.
  2. Navigate to "Appearance" in the left-hand menu and click on "Themes."
  3. Click on the "Add New" button at the top of the page.
  4. You will see a list of popular themes, or you can click "Upload Theme" to upload a theme from your computer.
  5. If you choose a theme from the list, you can preview it first by clicking on the "Preview" button. If you like it, click the "Install" button to install it.
  6. If you choose to upload a theme, click "Choose File" to select the theme file from your computer, then click "Install Now".
  7. Once installed, click the "Activate" button to activate the new theme.

If you installed a plugin as a theme, it could also cause the "style.css not found" error. This is because plugins and themes have different file structures, and WordPress expects certain files to be in specific locations based on whether they are a plugin or a theme.

The error message you encountered while trying to install the theme/plugin package is likely due to the fact that it was in .rar format. WordPress only supports installation of themes/plugins in .zip format, so you would need to extract the contents of the .rar file first before installing the theme/plugin.

To do this, you can use a file extraction tool such as WinRAR or 7-Zip to extract the contents of the .rar file on your computer. Once the files have been extracted, you should see a folder containing the theme/plugin files in .zip format. You can then upload and install this .zip file through the WordPress dashboard just like any other theme/plugin.