Theme development

Using Drupal 8, and customizing a subtheme based on Bootstrap 3.

When I click the mobile menu button (button element with the "navbar-toggle" class), the menu appears, but does it a bit too slowly; I need to reduce that time (or bring it to zero, need to see how it goes, to decide), but am unable to do so.

I've seen that when the button is clicked, the div of the menu

<div id="navbar-collapse" class="navbar-collapse collapse in" aria-expanded="true" style="">

hello I am using drupal 8.6.2 and every time I login I get the following error in the .theme file 
If I update the page the errors do not appear again while I'm in the session

Thank you very much for the help you can give me.

Hi, I'm new to drupal and I just installed a radix subtheme based on bootstrap 4. Everything was going fine but now the front page isn't displaying the titles of my articles. What can I do about that ?

Thanks.

Hello :)

How to add a class on all form in my theme. 

I want to add on all input type text a class for exemple form-input-text.

I have to rebrand and custom design the theme of a live and running Drupal 8 website. I wondered if there is any best practices post or blog that anybody could share or anybody would want to share their practices for such similar case.

I wanted to create a new sub theme on a suitable base theme.

In my limited experience I imagine if I want to design and test it in my localhost then I have to create a copy of the website in my localhost and then i would have to reinstall the website again on the live server.

For a small project, I'm working on a particularly not resource-efficient hosting, and I'd like to create a grid-organized home page layout (similar to the example in the image) without use of Views. I'm doing a lot of research, but I only find modules that partially solve the problem.
Can someone recommend something that can bring me closer to this result?
If not, I will be forced to use Views