In this tutorial I am going to create an OpenCart 3 custom theme named honcart.

STEP 1

Create a copy of theme folder at /catalog/view/theme/default and rename it to honcart

STEP 2

Make a copy of the following files

/admin/language/en-gb/extension/theme/default.php  
/admin/view/template/extension/theme/default.twig
/admin/controller/extension/theme/default.php

and then rename it to following

/admin/language/en-gb/extension/theme/honcart.php
/admin/view/template/extension/theme/honcart.twig
/admin/controller/extension/theme/honcart.php

STEP 3

Edit /admin/language/en-gb/extension/theme/honcart.php

Find

$_['heading_title']                    = 'Default Store Theme';

Replace

$_['heading_title']                    = 'Hon Cart Theme';

STEP 4

Open /admin/view/template/extension/theme/honcart.twig

Find

theme_default 

Replace

theme_honcart

STEP 5

Edit /admin/controller/extension/theme/honcart.php

Find

default

Replace

honcart

STEP 6

Change css link in header.twig file at /catalog/view/theme/honcart/template/common/

to

<link href="catalog/view/theme/honcart/stylesheet/stylesheet.css" rel="stylesheet">

STEP 7

Got to admin area > Extensions > Extensions > Choose the extension type > themes > install the theme

Then click the edit button and enable the theme > Status > Enable

STEP 8

Got to admin area > System > Settings > Edit store > Theme > Select Hon Cart theme > Save