First it's good to say that "professional looking" is subjective, so probably what it's better to look for is uniformity.
You are not too far honestly ...
First point is you are using lot of sections. If you use lot of sections in your page uncheck the "Collapsible" option, it will hide the top area of sections with the icon in the left. If you don't need 3-4 sections just use a unique one, no need of a section per element. Hide borders of your sections, so you don't see all the blocks in your page.
These few things will make it look better already.
Another point is style: keep colors, font, size etc. uniform in your objects.
You have blue, black, yellow, grey background on some of your rows, do you really need different colors? Do they really have a meaning? If not just use a unique one.
Start by this and keep in mind to make things uniform.
I have lot of sections because I have prompts for each section and I needed to give the condition for each section, to match the respective prompt selection. I tried to remove the border for the sections, the collapsible button is vanished but still the border exists.
I made some modifications to the Custom CSS style in both CSS class and CSS style as "display:hidden;" and the border vanished. Can you tell me if there is any standard color that OBIEE consultants should follow ?
There is no standard colors to use, it's like web design: any color can work as long as you keep it uniform and choose related colors.
There are multiple tools online giving you colors combinations which are supposed to work well together (based on a main color you choose).
If you want some ideas you can also read and find colors codes in the Alta Icon Web Design Guide : it's not a standard but it take you around different colors ideas, it can be a good place where to start.
You can also choose to adopt the same color guidelines of your employer/client: in general every company has a set of colors they use by default in Powerpoint presentaitons, Word templates, their website etc.
Thanks for your help