How to get rid of grid-flexbox.css

What problem are you trying to get around?

Here's an article that may help...  It's for Drupal but should be easy to implement in a custom module.

https://www.webomelette.com/im-hooked-7-using-hookcssalter

 

I would create a custom theme and not include bootstrap.

What theme are you using?

Have you checked out Layouts... they are very flexible for content.

 

The grid-flexbox.css file is coded in the Backdrop core in layout.module on line 924

I'm a programmer and I don't know how this information can be used, but it might be useful in diagnosing a problem and finding a solution.

Layouts and themes are treated separately in Backdrop.

You will need to create a new theme and a new layout.

Here's the .info file for a layout I created a while back...

type = layout
name = acsgrid
version = BACKDROP_VERSION
backdrop = 1.x

; Specify regions for this layout.
regions[header]         = Header
regions[headertop]      = Headertop
regions[headerq]          = Headerq
regions[headerq1]       = HeaderQ1
regions[headerq2]       = HeaderQ2
regions[headerq3]       = HeaderQ3
regions[headerq4]       = HeaderQ4
regions[headerbottom]     = Headerbottom
regions[top]            = Top
regions[content]        = Content
regions[third1]         = Tri First
regions[third2]         = Tri Second
regions[third3]         = Tri Third
regions[quarter1]       = Quarter First
regions[quarter2]       = Quarter Second
regions[quarter3]       = Quarter Third
regions[quarter4]       = Quarter Fourth
regions[bottom]         = Bottom
regions[footer]         = Footer

; The default region automatically receives the "Main Content" block if needed.
default region = content

; Preview
preview = acsgrid.png

; Include the Bootstrap4 Grid System
;libraries[] = bootstrap4-gs

; Include file for special preprocessing of this layout's variables.
file = acsgrid.php

; Default stylesheets for this layout
; stylesheets[all][] = acsgrid.css

and the layout file... layout--acsgrid.tpl.php

<?php
/**
 * @file
 * Template for the acsgrid layout.
 *
 * Variables:
 * - $title: The page title, for use in the actual HTML content.
 * - $messages: Status and error messages. Should be displayed prominently.
 * - $tabs: Tabs linking to any sub-pages beneath the current page
 *   (e.g., the view and edit tabs when displaying a node.)
 * - $action_links: Array of actions local to the page, such as 'Add menu' on
 *   the menu administration interface.
 * - $classes: Array of CSS classes to be added to the layout wrapper.
 * - $attributes: Array of additional HTML attributes to be added to the layout
 *     wrapper. Flatten using backdrop_attributes().
 * - $content: An array of content, each item in the array is keyed to one
 *   region of the layout. This layout supports the following sections:
 *   - $content['header']
 *   -  $content['Headertop']
 *   -  $content['Headerq']
 *   -    $content['HeaderQ1']
 *   -    $content['HeaderQ2']
 *   -    $content['HeaderQ3']
 *   -    $content['headerQ4']
 *   -  $content['headerbottom']
 *   - $content['top']
 *   - $content['content']
 *   - $content['third1']
 *   - $content['third2']
 *   - $content['third3']
 *   - $content['quarter1']
 *   - $content['quarter2']
 *   - $content['quarter3']
 *   - $content['quarter4']
 *   - $content['bottom']
 *   - $content['footer']
 */
?>
<div class="
 <?php print implode(' ', $classes); ?>"<?php print backdrop_attributes($attributes); ?>>
  <div id="skip-link">
    <a href="#main-content" class="element-invisible element-focusable"><?php print t('Skip to main content'); ?></a>
  </div>

 <header>
 
    <?php if ($content['headertop']): ?>
        <div class="l-headertop" aria-label="<?php print t('Site header top'); ?>">
      <div class="l-header-inner row">
        <div class="col12">
          <?php print $content['headertop']; ?>
        </div>
      </div>
        </div>
    <?php endif; ?>

    <?php if ($content['headerq1'] || $content['headerq2'] || $content['headerq3'] || $content['headerq4']): ?>
        <div class="l-headerq">
      <div class="l-headerq-inner row">
        <div class="l-headerq-region col-3">
          <?php print $content['headerq1']; ?>
        </div>
        <div class="l-headerq-region col-3">
          <?php print $content['headerq2']; ?>
        </div>
        <div class="l-headerq-region col-3">
          <?php print $content['headerq3']; ?>
        </div>
        <div class="l-headerq-region col-3">
          <?php print $content['headerq4']; ?>
        </div>
      </div>
    </div>
    <?php endif; ?>
  
    <?php if ($content['headerbottom']): ?>
        <div class="l-headerbottom row" aria-label="<?php print t('Site header bottom'); ?>">
            <div class="l-header-inner col12">
                <?php print $content['headerbottom']; ?>
            </div>
        </div>
    <?php endif; ?>
 
 </header> 
 
 <!-- /header -->
 
  <?php if (!empty($content['top'])): ?>
    <div class="l-top row">
    <div class="l-top-inner col12">
      <?php print $content['top']; ?>
    </div>
  </div>
  <?php endif; ?>

  <div class="l-wrapper">
    <div class="l-wrapper-inner row">

      <?php if ($messages): ?>
        <div class="l-messages" role="status" aria-label="<?php print t('Status messages'); ?>">
          <div class="l-messages-inner row">
            <div class="col12">
              <?php print $messages; ?>
            </div>
          </div>
        </div>
      <?php endif; ?>

      <div class="l-page-title">
        <a id="main-content"></a>
        <?php print render($title_prefix); ?>
        <?php if ($title): ?>
          <h1 class="page-title"><?php print $title; ?></h1>
        <?php endif; ?>
        <?php print render($title_suffix); ?>
      </div>

      <?php if ($tabs): ?>
        <nav class="tabs" role="tablist" aria-label="<?php print t('Admin content navigation tabs.'); ?>">
          <?php print $tabs; ?>
        </nav>
      <?php endif; ?>

      <?php print $action_links; ?>

      <!-- .l-middle -->
      <div class="l-middle row">
        <main class="l-content col-12" role="main" aria-label="<?php print t('Main content'); ?>">
          <?php print $content['content']; ?>
        </main>
      </div><!-- /.l-middle -->

      <?php if ($content['third1'] || $content['third2'] || $content['third3']): ?>
        <div class="l-thirds row">
          <div class="l-thirds-region col-4">
            <?php print $content['third1']; ?>
          </div>
          <div class="l-thirds-region col-4">
            <?php print $content['third2']; ?>
          </div>
          <div class="l-thirds-region col-4">
            <?php print $content['third3']; ?>
          </div>
        </div><!-- /.l-thirds -->
      <?php endif; ?>

      <?php if ($content['quarter1'] || $content['quarter2'] || $content['quarter3'] || $content['quarter4']): ?>
        <div class="l-quarters row">
          <div class="l-quarters-region col-3">
            <?php print $content['quarter1']; ?>
          </div>
          <div class="l-quarters-region col-3">
            <?php print $content['quarter2']; ?>
          </div>
          <div class="l-quarters-region col-3">
            <?php print $content['quarter3']; ?>
          </div>
          <div class="l-quarters-region col-3">
            <?php print $content['quarter4']; ?>
          </div>
        </div><!-- /.l-quarters -->
      <?php endif; ?>

      <?php if (!empty($content['bottom'])): ?>
        <div class="l-bottom row">
          <div class="l-bottom-inner col12">
            <?php print $content['bottom']; ?>
          </div>
        </div>
      <?php endif; ?>

    </div><!-- /.l-wrapper-inner -->
  </div><!-- /.l-wrapper -->

  <?php if ($content['footer']): ?>
    <footer class="l-footer row"  role="footer">
      <div class="l-footer-inner col12">
        <?php print $content['footer']; ?>
      </div>
    </footer>
  <?php endif; ?>

</div><!-- /.layout--acsgrid -->

Please note that this may not be 100% correct :-)  

But hopefully, it will give you an idea.

Hope this helps.