You can change the background color in the Customizer. For adding images, you will simply need to edit a page and add your images using the Media Library.

The easiest way to add HTML into a page is to switch your visual editor to text mode. Toggle to "Text" tab at the upper right corner of the editor window. Here, you can scroll down to the area you want to add your HTML and paste it in.

This appears to be more of a general WordPress issue than a BoldGrid specific issue, but I'd advise first checking your plugins through the "wp-content" directory (since you're unable to get into your site's Dashboard). You will need access the source files of your site using FTP or through your host control panel. Delete or rename the directory for any plugins you're not using. I'd advise contacting your hosting support if you have any trouble.

Unfortunately, BoldGrid does not have a randomization feature. You may want to investigate a different plugin or look into a self-coded solution. I'd recommend putting that script into a WordPress shortcode. A PHP developer can help you do that.

BoldGrid is a website builder tool with features that help design the look of a site. There are no SQL-specific features.

Bear in mind, picking an industry category from what's available is not required; these mainly provide example content. You can choose "Default" and start working from there. For a theme that comes with a default slider, you might like Hifidel or Primas.

Hifidel and Primas both have sliders by default, but sliders can be added to any theme.

Please provide some further context with regard to where and how you're trying to use this tag and the effect you're seeing. The Read More feature, as described by WordPress, should work as expected to in BoldGrid posts. 

Since this is not a known issue or general interface question, I'd advise submitting a support request via BoldGrid Central > Manage My Account > Support Request. This way, more information may be gathered and logged and possible bugs reported.

The best and safest way to customize a template is via the child theme system. This is basically a way of duplicating a BoldGrid theme into a new custom theme so you can make custom changes. If you're unfamiliar with HTML, PHP, CSS, or JavaScript, however, you would be best advised to get help from a developer or designer.

Unfortunately we're not equipped to provide development or design services upon request. In some rare cases, our agents have been able to provide some basic CSS as a general example, but providing free coding services is not something we're able to do in every case.