Summary:
Opencart 3 (currently) ships with Bootstrap 3.
This is mostly a tech preview of Bootstrap 4 with the hopes of adaption into future Opencart releases.
We love Opencart and Bootstrap and want to keep both of these beautiful pieces of software moving into the future!

* @category MonolithForge/OpenCart/Bootstrap4ThemeBasic
* @package opencart-bootstrap4-theme-basic
* @author dev@monolithforge.com
* @copyright 2018 Monolith Forge, LLC
* @license https://www.monolithforge.com/license/opencart-bootstrap4-theme-basic.txt
* @version 3.0.2.0-bootstrap4-theme-basic-a6

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Official Git URL: https://gitlab.com/MonolithForge/opencart-bootstrap4-theme-basic.ocmod

Mirrored Git URL: https://github.com/monolithforge/opencart-bootstrap4-theme-basic.ocmod

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NOTE: Currently in Alpha 7 dev mode.

DEMO LATEST: https://playground.opencartforge.com/u/m/monolithforge/demo/202/3.0.2.0/index.php

OTHER DEMOS:
Demo: https://playground.opencartforge.com/u/m/monolithforge/demo/202/3.0.0.0/index.php
Demo: https://playground.opencartforge.com/u/m/monolithforge/demo/202/3.0.0.2/index.php
Demo: https://playground.opencartforge.com/u/m/monolithforge/demo/202/3.0.1.0/index.php
Demo: https://playground.opencartforge.com/u/m/monolithforge/demo/202/3.0.1.1/index.php
Demo: https://playground.opencartforge.com/u/m/monolithforge/demo/202/3.0.1.2/index.php
Demo: https://playground.opencartforge.com/u/m/monolithforge/demo/202/3.0.2.0/index.php

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Installation Instructions

1. Make sure the apache user has permissions to write to the following directories:
chmod 0755 or 0777 admin/controller/extension/theme
chmod 0755 or 0777 admin/language/en-gb/extension/theme
chmod 0755 or 0777 admin/view/template/extension/theme
chmod 0755 or 0777 catalog/view/javascript/jquery/datetimepicker
chmod 0755 or 0777 catalog/view/javascript/jquery/datetimepicker/moment
chmod 0755 or 0777 catalog/view/theme

If 0755 does not work try 0777

2. Go to Extensions/Installer on left menu

3. Click on the upload button and upload the file opencart-bootstrap4-theme-basic.ocmod.zip
If there are errors, make sure step 1 was completed.
If step 1 was not completed correctly, go to Extensions/Installer and delete from history the opencart-bootstrap4-theme-basic.ocmod.zip and try again.

4. Go to System/Users/User Groups and click on Administrators (and/or other groups needing permissions for Bootstrap 4 Theme Basic).

5. Check Access Permission AND Modify Permission for extension/theme/bootstrap4_basic and click save.

6. Go to Extensions/Extensions and select Themes in the dropdown

7. On Bootstrap4 Basic Theme click the install button

8. On the same page click on the Edit button as we need to enable the theme.

9. Select Status Enabled and click save.

10. Click on System/Settings in the left menu.

11. Click on Your Store (default) (or your main store that you want to apply the theme to.)

12. On the General tab under Theme select Bootstrap4 Basic Theme and click save.

13. Go to your store and it should be in Bootstrap 4 ;-)

14. If step 13 didn't work, go to Dashboard and click on the Gear button on the top right of this page.
When the popup occurs, Click on both refresh buttons for Theme and SASS.

Extra Credit:
Look at the file catalog/view/theme/bootstrap4_basic/stylesheet/_variables.scss
Go to https://bootstrap.build and create different color schemes and change the content of your bootstrap css file located in catalog/view/theme/bootstrap4_basic/template/common/header.twig

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Changelog:

3.0.2.0-bootstrap4-theme-basic-a6
Fix Language dropdown.
Thank you wisamx for your help at: https://github.com/wisamx/opencart-bootstrap4-theme-basic.ocmod
aka: https://github.com/wisamx/opencart-bootstrap4-theme-basic.ocmod/commits/patch-1

3.0.2.0-bootstrap4-theme-basic-a5
Fix installation instructions

3.0.2.0-bootstrap4-theme-basic-a4
added summary and installation instructions to README

3.0.2.0-bootstrap4-theme-basic-a3
bootstrap-datetimepicker (tempusdominus/bootstrap-4) 5.0.0
-fixed remaining address custom date/time/datetime fields
Bootstrap 3 stateful buttons depreciated in 3.5.5 removed in v4
-fixed common.js add/remove cart button

3.0.2.0-bootstrap4-theme-basic-a2
update vendors:
bootstrap-datetimepicker (tempusdominus/bootstrap-4) 5.0.0
moment 2.22.1