You should not mess with definitions of classes like medium-up–one-third as these may be used somewhere else on your site and this change may brake that layout. (maybe does not really apply to your case as your rule applies to elements which has 3 classes at once, but first 2 of them are always used together anyway).
Instead, you should select proper classes that will apply at different screen widths.
medium-up–xxx classes apply at width 750px and up.
For screen widths less than 750 you should use small–xxx classes.
So, your collection element should look like:
which is actually a default’ish code for Debut theme.
You may see it defined in your collection-list.liquidSection like this:
...
{% case section.settings.grid %}
{% when 2 %}
{%- assign grid_item_width = 'medium-up--one-half' -%}
{% when 3 %}
{%- assign grid_item_width = 'small--one-half medium-up--one-third' -%}
{% when 4 %}
{%- assign grid_item_width = 'small--one-half medium-up--one-quarter' -%}
{% when 5 %}
{%- assign grid_item_width = 'small--one-half medium-up--one-fifth' -%}
{% endcase %}
{% for block in section.blocks limit: section.blocks.size %}
- ...
If your code differs, you can always download a fresh copy of the theme from the theme store as a draft theme for reference. However, it may be a different version as Shopify does update their themes now and then…