That is one way to solve it.
And well done with limited coding skills.
The problem is every time you add a new theme.
You will have to code another
latestimages_block.php to match the theme.
And the same goes for any other file you add, where you have the same issues.
Far easier to define all the classes in your
latestimages_block.php once.
Then create a section in your CSS theme file
Example
Code:
/*==Start All Defines For latestimages_block.php==*/
Then put all the define code here
/*==Finish All Defines For latestimages_block.php==*/
Then you just copy and paste that into each theme you have, and every new theme you add.
And just alter the colors to suit the theme.
Saves your root folder from been filled with loads of identical files.