Hi Rituraj,
Moodle 3.2.1+ includes some themes that should work okay - Clean, Boost, and More. There are others and if you go to Downloads > Moodle plugins directory, then in the search settings select Theme and Moodle 3.2 you should see all the themes that are the correct versions for the Moodle you have on your site.
When your equation appears much bigger than the normal text it seems to me it is probably being rendered by mimetex, which by default is set to render "Large" size. There are a couple of ways you can try to fix it. The one I recommend is to switch everything over to using MathJax. MathJax filter does a much better job of matching the equation font size to the rest of the text being used. The Large setting can be changed by editing one line of code at yourmoodle/filter/tex/pix.php at line 61 you should see:
$texexp = '\Large '.$texexp;
Try changing it to this:
$texexp = '\small '.$texexp;
You can also just comment out the line.
NOTE: If you alter the code you will have to remember to check/redo it every time you upgrade your Moodle site to a new version.
By the way, what delimiters are you using for your equations?