How do reduce?

looks like the white space you want to change is controlled in the colormag theme.

try something adding something like this – add as Custom CSS:

.cm-header-1 .cm-row {padding: 0 0;}

.cm-content {padding-top: 0px;padding-bottom: 0px;}

https://www.wpbeginner.com/plugins/how-to-easily-add-custom-css-to-your-wordpress-site/

for additional guidance about changing the theme, please start a topic for the theme’s support team https://ww.wp.xz.cn/support/theme/colormag/