Drupal 8.x

Hello, I have a issue with some values that I'm getting with hook_form_system_theme_settings_alter. The point is this:
with this hook, I want to create a color picker with:
$form['color'] = array(
'#type' => 'color',
'#title' => t('Color'),
'#default_value' => '#ffffff',
'#description' => t("choose a color")
);
}
then, on the admin panel, the admin introduce the value of the color in HEX. 
In a php file, I can obtain the color value with this:

Hello, let me explain what I'm trying to do:

I have an Event content type, which has a Datetime Range field (included in core) to specify the event start and end date time. I created a View where it lists those events paginated. I need a Contextual Filter where I specify in the URL a date (like 2019-02-28) and it'll filter all events that are occurring in this date. More technically, all events WHERE daterange_start <= filter AND daterange_end >= filter. (without considering times, only dates).

I have a successful, working install of 8.6.4 and am following the "manual upgrade" notes.  It says to put the site into "Maintenance Mode" but when I do I get an error that "The website encountered an unexpected error. Please try again later."

Anyone know what is going on?  I'm wondering if it's somehow a permissions thing and I need to make myself a higher prived admin...but haven't a clue why I'm not already as high as it goes.

One of the Drupal entities is Aggregator which comes with a defined set of how to save these entities using Drupal\Core\Entity\Sql\SqlContentEntityStorage

Would you agree with me that while I can alter the MySQL table that it would be ultimately self-defeating since it would be overwritten in the next Drupal core update? And better to alter it in a custom module?

With the new paradigm in Drupal 8, I don't know enough to say this is the best path? thx, sam

My view lists entities of a certain type and I need to let authorized users be able to run CRUD operations on them. Searching I find a suggestion that in D7 see how Drupal uses the Views API to do this. Makes sense, I would use that API for my custom module in D7. Now for D8, I suspect that the views would be a PlugIn.

Looking at /admin/content I understand that is a View of nodes with the operations I need. Looking at Webprofiler I see that the controller is not within Node, but within the Views module itself. So already my understanding is broken in D8.

I installed 8.6.4 a few months ago and there've been a number of versions of core that have come out since then.  I want to upgrade, but do I have to do it sequentially?  Like, first do 8.6.4 to 8.6.5, then to 8.6.6, etc., or can I just back things up, overwrite with 8.6.newest, and then copy back in the key parts of settings.php or .htaccess and it should work?

I have tried to get composer working and looked at drush, but for various reasons this isn't working.  Am opting to just do the manual upgrade -- but hoping I don't have to do this one by one by one.

Thank you!