simenatsu you can edit the value in the database, look for the settings table, and then the lines where key is theme_primary_color or theme_secondary_color, and edit the value column with the corrected value. Or use the following SQL to reset both columns to their default value:
UPDATE settings SET `value`='#4D698E' WHERE `key` = 'theme_primary_color' OR `key` = 'theme_secondary_color'
Then clear Flarum's cache with php flarum cache:clear to re-compile the CSS.